Field measurement of soil sorptivity and hydraulic conductivity

Transcription

1 Field measurement of soil sorptivity and hydraulic conductivity Item Type Thesis-Reproduction (electronic); text Authors Lien, Bob Kuochuan,1959- Publisher The University of Arizona. Rights Copyright is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 13/07/ :28:02 Link to Item

2 1 FIELD MEASUREMENT OF SOIL SORPTIVITY AND HYDRAULIC CONDUCTIVITY by Bob Kuochuan Lien A Thesis Submitted to the Faculty of the DEPARTMENT OF SOIL AND WATER SCIENCE In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA 1989

3 2 STATEMENT BY AUTHOR This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his or her judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from author. SIGNED: Li APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below:,(maal-x /2 1,Doci /ff? A. W. Warrick Date Professor of Soil and Water Science

4 3 TABLE OF CONTENTS LIST OF ILLUSTRATIONS 5 LIST OF TABLES 7 ABSTRACT 8 CHAPTER 1. INTRODUCTION 9 2. METHODS USED AND FIELD PROCEDURES 13 Cassel Ring 14 Disc Permeameter 17 Unsaturated Measurements with Disc Permeameter 17 Ponded Measurements with Disc Permeameter EXPERIMENTAL LOCATION AND DESIGN 27 Experimental Location 27 Experimental Design CALCULATION OF SORPTIVITY AND DISCUSSION 32 Calculation of Sorptivity 32 Analysis of Variance CALCULATION OF HYDRAULIC CONDUCTIVITY AND DISCUSSION 47 Calculation of Hydraulic Properties 47 Analysis of Variance ALTERNATIVE CALCULATIONS 63 Calculation Based on Jarvis's Model 63

5 4 TABLE OF CONTENTS (continued) Calculation Based on Two Tensions 7. SUMMARY AND CONCLUSION APPENDIX A UNSATURATED VERSION OF DISC PERMEAMETER CONSTRUCTION PROCEDURE APPENDIX B PONDED VERSION OF DISC PERMEAMETER CONSTRUCTION PROCEDURE 86 REFERENCES 91

6 5 LIST OF ILLUSTRATIONS Figure 2.1 Disc permeameter for unsaturated measurement Disc permeameter for ponded measurement ' Field map of Maricopa Agricultural Center Location of experimental sites. Coordinates are expressed as meters from the southwest corner of each field Sorptivity is equal to the slope of the cumulative infiltration vs. t regression line for small times Relationship between the variance and the treatment mean of sorptivity (a) before logarithmic transformation (b) after logarithmic transformation Sorptivity range by different methods at different sites Illustration of interaction between methods and sites for the sorptivity The steady-state flow rate is the slope of the cumulative infiltration vs. time regression line at large times Range of Ko by different methods at different sites Illustration of relationship between mean hydraulic conductivity and variance (a) before (b) after logarithmic transformation Interaction of experimental methods and sites for the hydraulic conductivity. 61 % 6.1 The plot of I/t* vs. t gives sorptivity as the intercept and hydraulic conductivity as the slope 63

7 6 LIST OF ILLUSTRATIONS (continued) Figure 6.2 Correlation of S o and S o after discarding data sets with negative K: and low r Correlation of Ko and,k0 after discarding data sets with negative Ko and low r 2. 69

8 7 LIST OF TABLES Table 3.1 Soil properties of the experimental sites Sorptivity results Correlation of S o with soil properties Coefficient of variation for method used in sorptivity measurement Analysis of Variance of sorptivity (mm/s 1/2 ) Duncan's Multiple Range Test of sorptivity (mm/s''2) Steady-state flow rate, soil moisture content, macroscopic capillary length and characteristic mean pore size Correlation of soil properties with characteristic mean pore size and K Hydraulic conductivity results Analysis of Variance of Ko (cm/h) Analysis of Variance of Koot (cm/h) Sorptivities and hydraulic conductivities by different calculation models Saturated hydraulic conductivities and alpha by Eg.(6.1] 72

9 8 ABSTRACT Four methods were applied at four experimental sites following a two-factor completely randomized design for field soil infiltration measurements at the University of Arizona Maricopa Agricultural Center. The Cassel ring and the disc permeameter at a 2 cm positive head provided saturated measurements whereas the 10 cm and the 5 cm tension disc permeameters provided unsaturated measurements which excluded pores ^ 0.03 and 0.06 cm in diameter, respectively. Sorptivity, hydraulic conductivity and characteristic mean pore size were calculated by the method given by White, Sully and Perroux (1989). Both sorptivity and hydraulic conductivity showed dependence on the method applied. The high sorptivity and hydraulic conductivity values obtained by saturated measurements were associated with the unavoidable presence of root channels and cracks at field hence provided large variation and poor repeatability. On the contrary, the disc permeameter at 5 cm tension demonstrated reliable repeatability and reasonable results.

10 9 CHAPTER 1 INTRODUCTION Application of soil science to agricultural practices, such as irrigation, drainage, infiltration and runoff control, groundwater recharge, as well as soil and water conservation, depends upon knowledge of the relevant hydraulic characteristics of the field soils. In recent years new techniques (Clothier and White,1981; Shani et al.,1987; Constantz and Murphy,1987; Ankeny et al.,1988; van Es et al.,1987; Wosten and van Genuchten, 1988 ; White and Perroux,1988; White, Sully and Perroux,1989) have resulted in the development of more precise, simple and reliable methods for flow phenomena and pertinent soil parameters determinations. Measuring in situ infiltration of field soils is a time consuming process. Van Es et al. (1987) proposed a field method with an one-dimensional flow device referred to as a "Cassel ring" to measure infiltration of field soils. Data obtained from each infiltration measurement can be analyzed in terms of sorptivity. The advantages of the Cassel ring method are that it is simple, rapid, inexpensive, and the requirement of soil volume for in situ infiltration

11 10 measurement is relatively small. Van Es et al. (1987) reported successful results for evaluation of infiltration variability of eroded fields. Perroux and White (1988) described the use of the disc permeameter, a modification of the sorptivity instrument of Clothier and White (1981), for the measurements of hydraulic properties of field soils containing macropores and preferential flow paths. The method for determining soil hydraulic properties from disc permeameter measurements was then approached by White, Sully and Perroux (1989) and was based on Wooding's (1968) three-dimensional flow from a shallow circular pond or surface disc. Data obtained from each infiltration measurement by any supply water potential can be analyzed in terms of the sorptivity and in terms of the steady-state flow rate. Thus, with the information of the soil moisture contents, the hydraulic conductivity and the macroscopic capillary length can be calculated. The advantages of the disc permeameter method are that it is simple, efficient and requires a small amount of water. In addition, with selectable water potential, the size of pore sequences or fissures which participate in the flow process can be dictated. Perroux and White (1988) stated that the applications of the disc permeameter method are particularly

12 11 useful in soil management and land degradation studies. Jarvis et al.(1987) followed the description by Collis- George (1980), based on Philip's (1969) two term equation, provided a simple and direct method of sorptivity and effective hydraulic conductivity calculation by the plot of infiltration data. The method does not require information of soil moisture contents for the calculation. However, the application is inappropriate if the steady-state infiltration condition was not developed within the period studied. The objectives of this experiment are: 1. Apply the Cassel ring method reported by van Es et al. (1987), and the disc permeameter method by Perroux and White (1988) for field soil infiltration measurements. 2. Use the calculation methods issued by White, Sully and Perroux (1989) to determine sorptivity, hydraulic conductivity, and macroscopic capillary length from the infiltration data obtained. 3. Corroborate the above results with the computational approach of Jarvis et al. (1987).

13 12 The experimental instruments for this research were constructed and machined in the Soil and Water Science (and Agriculture Engineering) shop at the University of Arizona Campbell Avenue Farm by Steve Evett and the author himself. All the instruments were developed by modifications of the devices proposed by van Es et al. (1987) and Perroux and White (1988). The field work was conducted between April 29 and June 17, 1989 at the University of Arizona Maricopa Agricultural Center (MAC). Cotton was planted and growing on the experimental fields during that period of time.

14 13 CHAPTER 2 METHODS USED AND FIELD PROCEDURES Methods used include: (1) Cassel ring, (2) disc permeameter at 10 cm tension, (3) disc permeameter at 5 cm tension and (4) disc permeameter at 2 cm positive head. The Cassel ring method and the ponded version of the disc permeameter provided saturated hydraulic properties measurements whereas the 10 cm and 5 cm tension disc permeameter provided unsaturated hydraulic properties measurements which excluded pores 0.03 and 0.06 cm in diameter, respectively, from the flow transport process. These eliminated pore radii (cm) are derived from the capillary equation (Watson and Luxmoore, 1986) that r = -2acosa/pgh r4-0.15/h [2.1] where a is the surface tension of water (cm/sec2 ), a is the contact angle between the water and the pore wall (assumed zero), p is the density of water (g/cm3 ), g is acceleration due to gravity (cm/sec 2 ), and h (cm) is the water supply potential provided by the disc permeameter. In order to minimize the effect of water quality on the infiltration process, tap water from the headquarters of MAC farm was used throughout this experiment.

15 14 Cassel Ring The Cassel ring was first developed and used by van Es et al.(1987) for sorptivity measurements. The ring used by van Es et al. is an open-ended metal cylinder approximately cm long and 10 cm inside diameter, whereas the Cassel ring used for our experiment is 24 cm long and 11 cm inside diameter cut from electro-mechanical tubing (EMT). The outside lower end of the ring was beveled to ease soil penetration when the ring was driven into the soil. A slide hammer driver was constructed to eliminate rocking and destruction of soil structure. The ring is forced into the soil 15 cm to ensure the wetting front will not descend past the lower end of the cylinder while the water is applied thus ensuring one-dimensional flow. Seven hundred and twenty-five ml of water is applied for an initial ponded head of 7.6 cm. The hydraulic head at the soil surface, which decreases with time, is assumed to have little effect on the infiltration event early in the infiltration process (van Es et al., 1987). Data was obtained by measuring the depth of water infiltrated versus time.

16 15 Following is the field procedure for the Cassel ring: 1. Use a shovel to clear out a smooth, level area about 50 cm long and 20 cm wide on the experimental location. In order to conduct the measurement under a uniform soil moisture content condition, cut away the ridge top of the soil surface until moist soil is revealed. Be careful not to compact the soil. 2. Place the Cassel ring on the prepared soil surface with a marked soil sample can aside. Place the lid of the can over its bottom, gently and vertically press the can all the way down into the soil until its bottom is just flush with the soil surface. 3. Drive the Cassel ring vertically into the soil using a slide hammer driver until the 15 cm mark on the outside wall of the ring is even with the soil surface. 4. Clip a millimeter scale ruler vertically on the inside wall of the ring with a position opposite to the sun. Place a "scotch" pad inside the ring to protect the soil surface from the falling water when water is applied. 5. Fill a 1000 ml volumetric cylinder with water to the 725 ml mark as precisely as possible. Make sure the "lap memory" stopwatch is reset and ready for use. 6. Lay down on the ground with a position so that your head is between the ruler and the sun but not blocking the

17 16 sun. Rapidly but smoothly pour water from the volumetric cylinder into the ring while starting the stopwatch simultaneously. Quickly adjust the position of your head until the reflection of the sun on the meniscus which meets the ruler scale is visible. The reflection of the sun appears as bright dot(s) when the sky is clear, and will appear as a very mild light line when cloudy. 7. Press the 'split' button to record the first time reading as soon as the reflection of the sun coincides with a millimeter mark. Continue recording the time readings for each subsequent millimeter mark until at least six minutes have passed. The initial water height at time zero can be back-calculated by a developed computer program. 8. Recall the recorded time readings from the stopwatch and enter them as well as the cumulative water height into a data sheet. Use a shovel to carefully remove the soil sample can from the soil for the initial moisture content and the bulk density determination. Return the site to as close to original condition as possible.

18 17 Disc Permeameter The disc permeameter was first built in 1982 and developed by K.M. Perroux (Perroux and White, 1988) by modification of the sorptivity tube design published by Clothier and White (1981). The device is designed to measure the in situ hydraulic properties of field soils containing macropores and preferential flow paths. It enables rapid measurement of sorptivity, hydraulic conductivity, macroscopic capillary length, and characteristic mean pore size with minimal soil disturbance (Perroux and White, 1988). Unsaturated Measurements with Disc Permeameter The construction of the permeameter was from the description of Perroux and White (1988). Whereas the device of Perroux and White for unsaturated measurements (negative water potentials) was machined from rigid plastic, our device was cast from clear polyester resin which allowed quick fabrication once an appropriate set of molds was designed and machined. The procedure for making the permeameter is in Appendix A. The bottom shallow reservoir was designed as a cone shape thus eliminating the accumulation of water bubbles. The flat bottom of the disc permeameter is a 20 cm inside diameter circular base covered by a fine mesh nylon membrane. The membrane is supported by a wire mesh backing and two

19 18 layers of porous interfacing material. The interchangeable reservoir and the tension tower of acrylic tubing are attached to the disc. The reservoir we selected for the -10 cm suction method was 3.2 cm inside diameter, and for the -5 cm suction method was 4.5 cm inside diameter. The tension tower provides the supplied tension and a pathway for air entering the reservoir as infiltration proceeds. A 25.4 cm inside diameter and 0.3 cm thick steel ring was constructed for the surface contact material which provides an excellent contact all across the base of the permeameter. The inside area of the sand filled ring served as the source area in our case. We chose our contact material following the suggestions of Perroux and White (1988). It was a Superstition sand soil which consisted mostly of medium and fine sand (96.3 % sand, 1.7 % silt, 2.0 % clay, 9.8 % VF sand, 30.1 % F sand, 39.5 % M sand, 16.0 % C sand, 0.9 % VC sand). The version of the disc permeameter for unsaturated measurement is shown in Fig Data was obtained by measuring the depth of water infiltrated versus time.

20 19 111, Interchangeable water reservoir nn.nn n n Air inlet nn em. vowill fa... n e- - MI= nnn OM. am. o mum* Bubble tower n n am. n n Om...mo ow. n 4=00 4= n n Ow* V. nn ma. ww0 a.m. 1 NO MOO n Om. e l NOY, Steel ring 25.4 Figure 2.1 Disc permeameter for unsaturated measurement

21 20 Following is the field procedure for the negative head disc permeameter: 1. Prepare a smooth, level area about 80 cm long and 50 cm wide on the experimental location. Be careful not to compact the soil. 2. Place a steel ring (0.3 cm thick, 25.4 cm ID.) on the center of the prepared location by pressing its four feet into the soil. The soil surface should be level enough so that the steel ring can mount on it perfectly. Press soil sample cans for initial moisture content into the soil on both sides of the steel ring, with each can about cm away from the ring. Completely fill the inside area of the ring with the sand contact material, and level it to the top edge of the ring by using a ruler as a straight edge. Remove any excessive sand material which falls outside the ring. 3. Remove the disc permeameter from the water bucket. Gently blow air into the air inlet tube of the tension tower and clamp it shut while it is bubbling to eliminate water inside the air inlet tube. Move the air inlet tube up or down in the stopper to make any changes of the tension. The provided tension is equal to the distance between the membrane and the air bubble entry point of the reservoir (which is 1.5 cm in our case) minus the

22 21 height of water above the bottom of air inlet tube. Fill the reservoir tube with water by using a suction pump to suck air out from the air tube at the top of the reservoir. 4. Read and record the initial water height on a data sheet. Enter the predetermined depth marks to be read on the data sheet. Unclamp the air inlet tube of the tension tower. Gently place the disc permeameter on the center of the sand covered steel ring. Start the stopwatch as soon as the bubble start to form in the tension tower. Press the 'split' button to record the time readings whenever the meniscus reaches the subsequent predetermined depth mark. Be sure to start writing down the time readings on the data sheet before 29 laps have been used since the stopwatch has only 30 laps. Press the 'stop' button when the meniscus reaches the last depth mark or at least when one hour's reading has been taken. 5. Wash away the sand material attached on the membrane before returning the permeameter to the water bucket. Use a spatula to scrape away sand material in the steel ring, and collect the top 1-2 mm depth soil sample for the final moisture content determination.

23 22 6. Recall and record the time readings stored in the stopwatch on the data sheet. Use a shovel to carefully remove the two soil sample cans from the soil for the initial moisture content and bulk density determination. Return the site to as close to original condition as possible. Ponded Measurements with Disc Permeameter Our version of the ponded-head permeameter followed the design of Perroux and White (1988). Perroux and White used a clear polycarbonate sheet while our 20 cm diameter disc was cast from clear polyester resin ( see Appendix B for more detail ). The interchangeable side tube and the reservoir of acrylic tubing are attached to the disc. The reservoir we chose for the +2 cm ponding method is 7.0 cm inside diameter. The side tube provides the volume of water required initially to fill the pond and a pathway for air entering the reservoir as infiltration proceeds. The holes at the bottom of the disc are covered with fine window mesh to hold water in the tube before the start of measurement. Three hooks with adjustable screws were built on the disc to enable the adjustment of the supplied potential. A triangular stand was made for the permeameter to set on. A 22 cm inside diameter sheet metal

24 23 20 cm 22 cm Figure 2.2 Disc permeameter for ponded measurement

25 24 ring was constructed to rest on the soil surface to confine ponded water. The version of the disc permeameter for ponded measurements is shown in Fig Data was obtained by measuring the depth of water infiltrated versus time. Following is the field procedure for the positive head disc permeameter: 1. Prepare a smooth, level area about 100 cm long and 50 cm wide on the experimental location. Be careful not to compact the soil. 2. Drive the triangular stand vertically into the soil until the stoppers on the legs of the stand meet the soil surface. Place soil sample cans for initial moisture content on both sides of the stand (about 20 cm away from the stand), and push them vertically into the soil until its bottom is just flush with the soil surface. 3. Place the empty permeameter on the triangular stand by hanging the hooks over the frame of the stand. Set the supply potential by adjusting the screws above the hooks. Measure the distance between the soil surface and the bottom of the disc to confirm the potential setting. Make sure the permeameter is level.

26 25 4. Lift the permeameter up, and slide the sheet metal ring into a proper position on the soil surface such that the permeameter is about at the center of the ring. Seal the ring on the outside with local soil, and wet the soil with a squeeze bottle. 5. Return the permeameter to the water bucket, and fill the reservoir tube with water by using a suction pump to suck air out from the air tube at the top of the reservoir. Also fill the side tube with water to meet the calibration mark on the outside wall of the side tube. The volume of water in the side tube is the amount of water necessary to fill the ponding area inside the ring (the calibration could be done in advance). 6. Carefully place the permeameter back in the ring. Read and record the initial water height on the data sheet. Enter the predetermined depth marks to be read on the data sheet. Un-clamp the air inlet tube. Start the stopwatch when the side tube empties. Press the 'split' button to record the time readings at each predetermined depth mark. Check for leaks on the seal periodically. Stop taking readings when the meniscus reaches the last depth mark or for at least one hour. Recall and record the time readings on the data sheet.

27 7. Remove the permeameter and the sheet metal ring, and collect the top 2-3 mm soil sample off the surface as soon as the free water disappears from the surface. Use a shovel to remove the initial moisture content/bulk density soil samples from the soil. Cut away the excess soil with a spatula. Cap the can and seal it with electrical tape. Return the site to as close to original condition as possible. 26

28 27 CHAPTER 3 EXPERIMENTAL LOCATION AND DESIGN Experimental Location The study area is the University of Arizona Maricopa Agricultural Center located three miles east of Maricopa and three miles north of the Casa Grande/Maricopa Highway in Pinal County, Arizona. The farm is 770 hectares in size and the elevation is 358 meters. The dominant soils at the farm are Casa Grande (fine-loamy, mixed, hyperthermic Typic Natrargids), Shontik (fine-loamy, mixed, hyperthermic Natric Camborthids), and Trix (fine-loamy, mixed (calcareous), hyperthermic Typic Torrifluvents) (Post et al.,1988). The experiments were conducted at four sites (20,40,100,120) selected from field 28 and field 29 (Figure 3.1 & 3.2). Site 20, located at field 28, is 665 m east and 150 m north from the south-west corner of the field. Site 40, also in field 28, is 1365 m east and 50 in north from the south-west corner of the field. Site 100 and site 120 are chosen from field 29, they are 465 in east and 80 in north, 1465 m east and 80 in north, respectively, from the south-west corner of the field 29. The dominant soils at the experimental sites are Casa Grande and Trix.

29 'wow, A F r A * FIELD 29 * FIELD mi Figure 3.1 Field map of the Maricopa Agricultural Center

30 29 (H o 4-1 o g a) +) 0

31 30 Soil samples from the top 0-30 cm 'Ap' horizon of each site were previously taken to determine the soil particle size distributions. Bulk density soil samples were taken during the experiment. The results are shown in Table 3.1. Table 3.1 Soil properties of the experimental sites Site Gravel Sand Silt Clay Bulk soil No. (Z) (%) (Z) density texture sandy clay loam silty clay clay loam sandy loam Experimental Design A completely randomized design (Gomez and Gomez, 1984) was used in each site to minimize the effects of spatial bias. An assumption was made that the soil texture within a site is uniform compared with the large scale of the field. There were 4 methods (Cassel ring, disc permeameter with -10 cm tension, disc permeameter with -5 cm tension, and disc permeameter with +2 cm head) with five replications for each method. The area of each site was 5 x 5 m2 with 5 rows and 4

32 columns facing the east. The overall experimental design was 31 a completely randomized design with two factors -- method and site. The treatment variation and experimental error can be detected to determine if the difference between treatments is real or is due to chance. Moreover, the interaction of the methods and sites can be also examined.

33 32 CHAPTER 4 CALCULATION OF SORPTIVITY AND DISCUSSION Calculation of Sorptivity Sorptivity, described by Philip (1969), is a measure of the uptake of water by soil without gravitational effects. It can be calculated from the cumulative infiltration data collected in the early infiltration event based on the infiltration equation given by Philip: I = St 1/2 + At + Bt3/2 + [4.1] where I is cumulative infiltration (mm) at time t (sec). S, A and B are coefficients. The coefficient, S (mm sec -1/2 ), is defined as sorptivity which is equal to the slope of the cumulative infiltration versus the square root of time for small times (Fig.4.1) SQRT(T) (s'0.5) Figure 4.1 Sorptivity is equal to the slope of the cumulative infiltration vs. t 1/ regression line for small times.

34 33 Philip (1969) contended that during the early stages of flow at short infiltration times, the flow system behaves as if it were one-dimensional and is dominated by capillarity. Talsma (1969) further reported that the influence of gravity on one-dimensional infiltration is negligible for t t gray, with t grav = s02/ (K0 Kn) 2 Here S o is the sorptivity at the water supply potential * 0, Ko and Ko are the hydraulic conductivity at supply potential * 0 and initial water potential respectively. White and Sully (1987) found that t grav varied between 0.08 and 34 h in the field, thus the linearity of i vs. t112 implied by Eq.[4.1] was expected to hold for as little as 6 s to as long as 2450 s. In our case, the existence of linearity was examined by the r 2 value of the I versus.0 plot at short infiltration time, and the sorptivity was determined by using the first 5 or 7 infiltration data points for the Cassel ring method and the ponded disc permeameter. For the unsaturated disc permeameters, the first few infiltration data were omitted to negate the presaturation process of the sand contact material. All the infiltration data eliminated was less than two minutes.

35 34 Table 4.1 Sorptivity results Site,Method & (Rep.) Sorptivity (ram/s34) r2 Mean S.D. % CV 20 Cassel(1) Cassel(2) Cassel(3) Cassel(4) Cassel(5) Cassel(1) Cassel(2) Cassel(3) Cassel(4) Cassel(5) Cassel(1) Cassel(2) Cassel(3) Cassel(4) Cassel(5) Cassel(1) Cassel(2) Cassel(3) Cassel(4) Cassel(5) disc-10(1) disc-10(2) disc-10(3) disc-10(4) disc-10(5) disc-10(1) disc-10(2) disc-10(3) disc-10(4) disc-10(5) disc-10(1) disc-10(2) disc-10(3) disc-10(4) disc-10(5)

36 35 Table 4.1 (continued) Site,Method Sorptivity & (Rep.) (mm/&) r2 Mean S.D. % CV 120 disc-10(1) disc-10(2) disc-10(3) disc-10(4) disc-10(5) disc-5(1) disc-5(2) disc-5(3) disc-5(4) disc-5(5) disc-5(1) disc-5(2) disc-5(3) disc-5(4) disc-5(5) disc-5(1) disc-5(2) disc-5(3) disc-5(4) disc-5(5) disc-5(1) disc-5(2) disc-5(3) disc-5(4) disc-5(5) pond+2(1) pond+2(2) pond+2(3) pond+2(4) pond+2(5) pond+2(1) pond+2(2) pond+2(3) pond+2(4) pond+2(5)

37 36 Table 4.1 (continued) Site,Method Sorptivity & (Rep.) (mm/s/) r2 Mean S.D. % CV 100 pond+2(1) pond+2(2) pond+2(3) pond+2(4) pond+2(5) pond+2(1) pond+2(2) pond+2(3) pond+2(4) pond+2(5) The sorptivities of the infiltration measurements from the four experimental sites were calculated by Eq.[4.1] with r 2 ^ 0.96 (Table 4.1). It was found that the linear relationship of the calculated S o is insignificant with both soil texture and bulk density (Table 4.2). Note that the insignificant linear relationship does not necessarily imply an absence of non-linear relationships. However, meaningful examinations of other non-linear type of correlation were confined by the small degrees of freedom. The statistics of sorptivities by different sites and methods are presented in Table 4.1. The disc permeameter at 5 cm tension had the narrowest range of coefficient of variation (15.38% %) while the Cassel ring method delivered the largest range of coefficient of variation (15.74% %).

38 37 Table 4.2 Correlation of S o with soil properties. X Y Corr(r) Slope(b) Y Int(a) d.f. P S o %Sand ns S o %Silt ns S o %Clay ns S ob.d ns ns not significant Table 4.3 Coefficient of variation for method used in sorptivity measurement. method site Min.; Max.S yo mean S w o (mm/s1/2 ) (minis) (mm/s ) S.D. % CV Cassel all Disc-10 all Disc-5 all Pond+2 all The disc permeameter at 10 cm tension and at +2 cm positive head provided a %CV range of 14.51% % and 14.45% %,respectively. The overall coefficient of variation for each single method was also computed (Table 4.3). The Cassel ring method had the highest value of %CV. The disc permeameter at 10 cm tension resulted in the second largest %CV. And the disc permeameter at 5 cm tension and the disc permeameter at +2 cm pond gave relatively smaller coefficient

39 of variation. Thus, %CV was not increased by the increase of water supply potential as obtained by Watson and Luxmoore (1986). Surprisingly, it was found that the overall %CV of the +2 cm ponded method gave the lowest %CV (21.3%) value among the methods. The widest range of %CV by the Cassel ring method suggested that the method is less pertinent than the others for the sorptivity measurement. The repeatability of the Cassel ring method is considered to be poor according to the generally larger %CV value. On the contrary, the disc permeameter at 5 cm tension yielded a consistently lower %CV which could lead to the conclusion that the disc permeameter at 5 cm tension is the most appropriate and the most repeatable method for the sorptivity measurement in this experiment. Perroux and White (1988) found a similar result of considerable larger coefficient of variation of sorptivity for water supply potential ^ -4 cm and concluded it was due to macropores or preferential flow paths. However, our +2 cm positive head disc permeameter method does not show a noticeably larger %CV than those of the negative head disc permeameter. The result suggests that the variation of sorptivity for water supply potential -5 cm in our experiment might be larger than what it should be and is due to the inherent soil variability or to measurement error. 38

40 39 Analysis of Variance An Analysis of Variance (ANOVA) was applied to compare the sorptivity results of different methods at different sites. The hypothesis for the analysis is no significant difference of the sorptivities measured regardless of the method applied and the experimental location although we were actually expecting the sorptivity variances to differ. Since our experimental methods provided different water potentials which control the size of macropores that participated in the capillary flow, the ANOVA still is appropriate because of its ability to detect whether variation of the sorptivity is due to the experimental scatter or real differences between treatments. If the treatment variation is significantly larger than the experimental error, then the differences between the treatments is said to be real and significant. As the basic assumption of ANOVA, logarithmic transformation procedure was utilized in our analysis to help satisfy the requirement of homogeneity of variances (Gomez and Gomez, 1984). As shown in Figure 4.2, sorptivity data before logarithmic transformation has a non-homogeneous variance distribution while data after logarithmic transformation is somewhat closer to homogeneous. Since the sorptivity data obtained are small (i.e. less than 10), log(x+1) was used

41 40 instead of log(x), where x is the original sorptivity data. The transformed data were used throughout the analysis of variance (Table 4.4) and the Duncan's Multiple Range Test (DMRT) (Table 4.5). The original (untransformed) mean is included as the final presentation of the DMRT since it represents more adequate interpretation of the result than the transformed mean. 0.3 (a) (b) 0 0 CO 0.2 CO > al a.) 0.03 o Fis o o t5 2 Mean (mm/e0.5) Mean (log transformed) Figure 4.2 Relationship between the variance and the treatment mean of sorptivity (a) before logarithmic transformation (b) after logarithmic transformation.

42 41 Results from the Analysis of Variance (Table 4.4) showed that the difference of sorptivities is significant at the 99% confidence level in experimental methods as well as their interaction, but insignificant with respect to sites. As shown in the Duncan's Multiple Range Test (Table 4.5), the Cassel ring method showed a significantly larger sorptivity than the others with an order of Cassel Ring > ponded disc permeameter > disc permeameter at 5 cm tension ^ disc permeameter at 10 cm tension. The arithmetic mean sorptivity for the Cassel ring method is 4 to 6 times that of the unsaturated sorptivity value. The arithmetic mean sorptivity for the ponded disc permeameter is 3 to 5 times that of the unsaturated conditions. These differences are likely brought about by the water supply potentials due to the influence of macropores and preferential paths on flow. Perroux and White (1988) obtained a threefold change of sorptivity from a 2.8 cm difference in water supply potential between ponded and unsaturated conditions. However, in our experiment, the difference of sorptivities between the two unsaturated conditions, which were equivalent to the exclusion of pore size ^ 0.03 cm and ^ 0.06 cm in diameter, was indicated statistically insignificant even though a mm/s Yz change in sorptivity per cm water supply potential does exist. Such a statistical result implied the contribution of

43 o (1) 0.5 o 0.9 \ E Site Site Disc Disc-10 fr.)- 0.4 P 0.3 \ E 03 E 0.2 o (i) High-Low X Mean Site High-Low X Mean MO 120 Figure 4.3 Range of S o by different methods at different sites. Site the pore size between cm in diameter on the early stage infiltration process was trivial. The sorptivity range by different methods at different sites is illustrated in Figure 4.3. It was found that the more infiltration is influenced by macropore flow, the larger the variation would be.

44 43 Table 4.4 Analysis of Variance of sorptivity (mm/ss4 ) Source SS df MS Main Effects method 2.093E E ** site 4.102E E ns Interaction method x site 2.084E E ** Error 3.520E E-5 Total 2.695E-2 79 ** highly significant different at 99% confidence level ns not significant different Table 4.5 Duncan's Multiple Range Test of sorptivity(mm/s Iv2 ) transformed original non-significant+ Rank method Mean Mean n Range 1 Cassel a 2 Pond b 3 Disc c 4 Disc c transformed original non-significant+ Rank site Mean Mean n Range a a a a + treatments follow the same letter are not significantly different at P < 0.05

45 Site Figure 4.4 Illustration of interaction between methods and sites for the sorptivity. The interaction was indicated significant at the 99% confidence level as shown in table 4.4 and illustrated in Figure 4.4. However, once the interaction between two experimental factors is present, the application of the Duncan's Multiple Range Test becomes tenuous. Although the ANOVA showed negligible effect of the experimental locations on sorptivity measurements, it is meaningless to clarify the

46 45 influence of experimental site to sorptivity directly by the use of DMRT and ANOVA. As illustrated in Figure 4.4, the unsaturated methods established homogeneous responses of sorptivity estimation to locations whereas the saturated method presented erratic responses of sorptivity determination. Therefore, the consideration of the effect of experimental location on the sorptivity result by the method of DMRT could be misleading by the existence of significant interaction. It is noticed that on site 100 (clay loam, bulk density = 1.15 g/cm3 ), the flow is perhaps dominated by macropores and preferential flow paths as evidenced by the small unsaturated sorptivity and the large saturated sorptivity. The statement is supported by Watson and Luxmoore's (1986) conclusion that at sites with a high ponded infiltration rate, the non-macropore flow accounts for a very low fraction of the total flow. We can conclude that at site 100 the use of the Cassel ring or ponded disc permeameter for the matrix sorptivity measurement is less suitable due to the existence of root channels or macropores which could lead to a significant variation for the result. The existence of macropores or root channels is evidenced by the relatively low bulk density value. On the contrary, site 120 with relatively high bulk density (1.42 g/cm 3 ) resulted in higher unsaturated flow and lower macropore flow. The erratic jump of sorptivity

47 46 value obtained by the ponded disc permeameter at site 40 could be attributed to the possible water leakage from the sheet metal ring.

48 47 CHAPTER 5 CALCULATION OF HYDRAULIC CONDUCTIVITY AND DISCUSSION Calculation of Hydraulic Properties The method for determining soil hydraulic properties from disc permeameter measurements in the field is given by White, Sully and Perroux (1989) and is based on Wooding's (1968) analysis of the three-dimensional flow from a shallow circular pond or surface disc. Wooding stated that when water is applied at a potential of * the steady state volumetric flow rate q is: q = 1rr0 2 (K0 - Kr) ) 4r0K0Ac [5.1] The first term on the right of Eq.[5.1] represents the contribution of gravity to the total flow from the surface disc and the second term represents the contribution of capillarity to the flow, where r, is the radius of the pond or sand cap area. In our case, the value of ro is 11 cm for the ponded version of disc permeameter and 12.7 cm for the unsaturated version of disc permeameter. K, is the hydraulic conductivity at the supply potential 11, 0, and Kn is the hydraulic conductivity at the initial soil water potential For relatively dry soil, Er is much smaller than Ko thus its effect can be ignored. A c is the macroscopic capillary

49 48 length, which is an inherent soil-based length scale of the relative magnitudes of the capillarity and gravitational components of soil-water flow. The larger A 0 the higher the contribution of capillarity to the flow transport process. White and Sully (1987) showed that A 0 is a function of the sorptivity and the hydraulic conductivity: A c = bs 02 / [ (8 0 - en )K0 ] [5.2] where 90 is the moisture content at the supply potential * 0, 8n is the initial moisture content at Wn, S o is the sorptivity at * n with supply potential *0 and b is a dimensionless constant which value lies between 1/2 and r/4. For field soil, White and Sully (1988) suggested that b 0.55 is a reasonable approximation for most situations. White and Sully further related A 0 by simple capillary theory to a characteristic mean pore size A m for pure water at 20 C by: A m = 7.4 / A c [5.3] where A m and A c are in mm. From Eq.[5.2], Eq.[5.1] can be rewritten as: q = rr02k0 + 4r0b502 / (80-8n ) [5.4] and the steady state flow rate per unit area is: q/rr02 = K0 + 4bS 02 / [rr0 (80 - en)) [5.5]

50 49 The value of q/rr: for large times can be found by plotting the cumulative infiltration. The slope of the linear plot at the large times is the steady state flow rate qhrr02 (figure 5.1). Therefore, for each field measurement, once the steady state flow rate, sorptivity, initial and final soil moisture content and the area of source were determined, hydraulic conductivity can be found by: Ko = 2 q/rr0-4bs 2 0 / prr0 (8 0-8 o) ] [5.6] Steady-state flow rate,1./ Site: 20 Rep: C3 o o t (sec) Figure 5.1 The steady-state flow rate is the slope of the cumulative infiltration vs. time regression line at large times.

51 50 The hydraulic conductivities of each individual experimental method, except the Cassel ring, were calculated by Eq.[5.6] (Table 5.3). The steady-state flow rate and the soil volumetric moisture content are shown in Table 5.1. The hydraulic conductivity of the Cassel ring method were estimated by assuming it is equal to the value of qhrr02 according to the assumption of the Green and Ampt equation. The disc permeameters at negative water supply potential gave unsaturated hydraulic conductivities at the supply potential * 0, whereas the Cassel ring and the positive head disc permeameter contributed the measurement of the saturated hydraulic conductivies where Ko = Ksat The unsaturated hydraulic conductivity can be converted into saturated hydraulic conductivity by assuming the exponential relationship between Ksat and K(111 0 ) that K(4,0) = KsatexP (*co/a ) The conversion value of saturated hydraulic conductivity are also presented in Table 5.3. The computed macroscopic capillary length and characteristic mean pore size are listed in Table 5.1. The Cassel ring method is not included because the final soil moisture content was not measured. The arithmetic mean of A c at site 20, 40, 100, 120 are 71.3 mm, 14.4 mm, 145 mm and 97.8 mm, respectively. The results indicate the contribution

52 Table 5.1 Steady-state flow rate, soil moisture content, macroscopic capillary length and characteristic mean pore size. 51 Site,method (Rep.) cyrr 2e ee Ac (cm/s) (cm/cm) (cm/cm) (mm) Am (mm) 20 disc-10 (1) 3.05E disc-10 (2) 2.90E disc-10 (3) 5.67E disc-10 (4) 5.42E disc-10 (5) 6.68E disc-5 (1) 9.15E disc-5 (2) 9.32E disc-5 (3) 5.99E disc-5 (4) 8.78E disc-5 (5) 1.11E pond+2 (1) 2.98E pond+2 (2) 3.26E pond+2 (3) 5.19E pond+2 (4) 4.75E pond+2 (5) 3.83E disc-10 (1) 2.20E disc-10 (2) 3.14E disc-10 (3) 2.23E disc-10 (4) 2.58E disc-10 (5) 2.60E disc-5 (1) 4.86E disc-5 (2) 5.67E disc-5 (3) 4.86E disc-5 (4) 6.48E disc-5 (5) 4.00E pond+2 (1) 3.01E pond+2 (2) 3.25E pond+2 (3) 4.35E pond+2 (4) 6.03E pond+2 (5) 4.58E disc-10(1) 1.18E disc-10(2) 1.30E disc-10(3) 1.81E disc-10(4) 1.43E disc-10(5) 1.00E

53 52 Table 5.1 (continued) Site, method (Rep.) q/n-r2 ( cm/ s ) A erl A A 90 A ( cm-/ cm- ) ( cm-/ cm- ) Ac ( mm ) Am (mm) 100 disc-5(1) 4.76E disc-5(2) 3.91E disc-5(3) 5.67E disc-5(4) 3.49E disc-5(5) 2.68E pond+2(1) 8.28E pond+2(2) 7.50E pond+2(3) 8.62E pond+2(4) 6.30E pond+2(5) 7.81E disc-10(1) 5.09E disc-10(2) 6.31E disc-10(3) 3.72E disc-10(4) 4.77E disc-10(5) 3.75E disc-5(1) 6.82E disc-5(2) 8.19E disc-5(3) 7.98E disc-5(4) 9.96E disc-5(5) 8.48E pond+2(1) 4.34E pond+2(2) 3.32E pond+2(3) 4.46E pond+2(4) 4.31E pond+2(5) 4.52E

54 Table 5.2 Correlation of soil properties with characteristic mean pore size and Ko. X Y Corr(r) Slope(b) Y Int(a) d.f. P A m % Sand ns A m % Silt ns Am % Clay ns A m B.D ns Ko % Sand ns Ko % Silt ns Ko % Clay ns Ko B.D ns 53 ns not significant of capillarity to the flow at site 40 as well as site 100 are greater than for site 20 and site 120. It is also found that the calculated A c value falls in an enormous range of coefficient of variation for 40.0% - 191% at the experimental sites. The arithmetic mean of A m were found mm, mm, mm and mm at site 20 (sandy clay loam), 40 (silty clay), 100 (clay loam) and 120 (sandy loam), respectively. Neither soil texture nor bulk density shows significant correlation with characteristic mean pore size (Table 5.2). The coefficient of variation for A m carries a wide range of 53.3% - 107%. The large coefficient of

55 54 variation value shows either the inherent soil properties are quite variable with experimental site or that the experimental error is enormous by using Eq.[5.2] and Eq.[5.3] for the estimation. Consequently, the reliability of these parameters is truly doubtful since the number was estimated from two miscellaneous functional terms S0 and K0 by Eq.[5.2], which could lead to a tremendous sensitivity of A c and A m determinations. As shown in Table 5.3, the Cassel ring method and the disc permeameter at 10 cm tension contributed inconsistency to the hydraulic conductivity estimation by giving higher value, as well as larger range of coefficient of variations. The range of calculated K0 by different methods at different sites is illustrated in Figure 5.2. The saturated measurements resulted in higher K0 as well as higher variation than for the unsaturated measurements. Note that the hydraulic conductivities by the Cassel ring method are actually determined by assuming it is equal to the steady