Hydraulic condition of subsurface drainage system (case study, Sari, Iran)

Transcription

1 International Journal of Agriculture and Crop Sciences. Available online at IJACS/2013/5-6/ ISSN X 2013 IJACS Journal Hydraulic condition of subsurface drainage system (case study, Sari, Iran) Z.Ahmadi 1, Gh. Aghajani 2 1. Former M. Sc. Student, Sari Agricultural sciences and Natural Resources university 2. Instructor, Sari Agricultural sciences and Natural Resources university Corresponding author aliponh@yahoo.com ABSTRACT: The downward flow to the drain pipe has a greater influence on the movement of soil particles toward drain envelope as compared with the horizontaland radialflow. In this study, by installing a singular subsurface drainage systemconsisting threedrain pipeswith drain spacing of 20 m and drain depth of 1.5 m, at approximately one hectare field of Sari Agricultural Sciences and Natural ResourcesUniversity, the effect of the elimination of downward flow to the drain pipe was investigated on the water table level and drainage flow. Prevention of direct entry of the vertical flow into drain pipe was performed by placing a layer ofplastic coveron the sand envelope of themiddle drain pipe. Water table level fluctuations were measured in holes which were dug in each drain trench and at 0.5, 1.5, 5 and 10 m spacing apart from each drain at intervals of 5, 15, 25, 35, 45, and 55 meters from collector ditch. Water table depth and drain discharge were measured from April 21 to December 21, The average of drainage discharge of no plastic drain (drain A) was more than that of plastic covered drain (drain B) about 12 % and there was a significant difference (p=0.05) between drainage water volume of drains A and B. The average depth of water table levels within the trench of drain A was 9.1 cm more than the corresponding water table depth of drain B. Also, the average depths ofwater table in the 0.5, 1.5, and 5 m spacing apart drain A were approximately 5.2, 2.9, and 0.05 m higher than of thosevalues of drain B. Sediment load of drain A was 74% more than that of drain B indicating the considerable role of the inflow to drain from its upper part on the sediment transport into drain pipes. Key words: Soil internal erosion, vertical flow, plastic cover, water table depth, sediment load. INTRODUCTION To produce required food of the increasing population of the world, it is necessary to increase the cultivatedland productivity or more lands to be cultivated. Predictions show that food production in the next 25 years should be doubled (Ritzema, 2007). The main part of this increase should be obtained by investment related to the improvement of irrigation and drainage practices in existing agriculture lands. One of the most important irrigation and drainage practices is land drainage. Of over 1500 hectares lands under cultivation (irrigated or rainfed) in the world only 14% are equipped with all types of drainage networks.predictions of crop production for food and forage needs in the next 25-year, show that, globally, drainage should be extended in at least million hectares area. It is forecasted that one third of this area should be equipped with subsurface drainage (National Irrigation and Drainage Committee, 2007). Drainage projects are faced with challenges in the various stages of research, design and implementation that their negligence led to the ineffectiveness of such projects.prevention of particles entrance into subsurface drains is done by usingporous materials called drainage envelopes. The main goal of these materials is infiltration improvement around drains (Ministry of Agriculture, 2000; Stuytetal, 2000). In addition, the drainage envelopes can improve the bed conditions (Bybordi, 1999) and reduce resistance against entrance flow into drains (Stuyt&Dierickx, 2006). The flow resistance in the vicinity of subsurface drains has high relationship with soil structure characteristicsincluding its coarsepores and geometry arrangement of coarse porous networks around the drains.

2 There is limited information about envelope effectson the water flow pattern toward subsurface drains.also, its effect on the radial and entrance resistances is also limited. Investigation of all patterns of water flow toward subsurface drains showed that envelope characteristics, in allcases, had not any significant effect on the geometrical shapes of flow patterns. It seems that change in the flow resistance in the vicinity of subsurface drains to a great extent depends on the soil characteristics, such as coarse pores and geometrical arrangement of coarse pores in the vicinity of drains (Stuyt and Dierickx 2006). RamazaniMoghaddamet al. (2009) investigated the efficiency of artificial envelope of Iranian factories by soil and sandy reservoirs and compared it with Netherland type. The results showed that change in the discharge in three first tests was high and then decreased to a constant limit.the comparison of water resistance around drainpipe and the amount of sediment entered into it showed that in both cases the Netherland envelope had the least amount.up to 16 th day since the beginning of the test, the water resistance in three artificial envelopes was same but in the 17 th day, its amount in Netherland envelope had a considerable decrease. Kabusiet al. (2006) studied the ability of rice husk as drainage envelope under laboratory test. Based on the results, the rice huskeven at high concentration, have high hydraulic conductivitydemonstrated good hydraulic workability of rice husk. Also, in comparison to sand and silt envelopes, the rice huskdecreased drainage water due to fine texture and spindle shape of rice husk, which increases water entrance resistance.karimiet al. (2007) evaluated the application of rice huskand concluded that after 10 years, the drainage system has fast interaction and the ground water level fell rapidly due to drainage. Dierickx (1980) studied hydraulic condition and particle movement in the vicinity of drains using a simple analogue model. The result showed that the most important parameter related to the design of drainenvelope is or the size of orifice, that 90% of particles are smaller than that and is called effective diameter. The movement of suspended soil particles from upper layer of drains toward drainage envelopes and finally into drain pipe, can accumulate sediment inside the drain pipe and decrease its useful capacity, clogging the pipe and drainage envelopes.continuous movement of suspended particles into drain pipe can make underground pores. By increasing the size of pores, the load on the soil above drain pipes, cause subsidence the soil and creation of great pores in the soils abovedrainpipes(national committee of Irrigation and Drainage,2004). The subsurface drainage systems, constructed by high costs, should provide the design expectations during theiruseful age. One of the most important problems causing negative effect on the application of thesesystems,is the problem of pipe clogging, and envelopes around the pipe.the aim of this research is introduction of an applied approach to decrease the problem, so that one can use better of the investments made on the performance of the drainage systems. MATERIALS AND METHODS This research was carried out in one hectare land of the research farm of Sari Agricultural Sciences and Natural Resources University. The latitude and longitude of the land are and 53.04, respectively. The area has MSL of -15m. A single drainage system consisting of 3 lines of drain with distance of 20m, average depth of 1.5 m was installed in the farm. PVC corrugated pipewith 100mm diameter was used as drain pipe. Sand and silt (mineral) envelopes with thickness of 10cm wereused around drain pipe. For prevention of direct entrance of vertical flow from above drain into drain, a layer of plastic was covered on the envelope of the middle drain. During sampling (April-December 2011) 37 rain events occurred and 466.9mm rainfall was measured. The least and highest monthly rainfall events was in June and Sept., with 6.6 mm and mm, respectively. The minimum and maximum temperatures were -1 and, respectively. A summary of weather data of the site was shown in Table 1. To investigate change in water table depths in different times, auger holes were dug in 0.5, 1.5, 5 and 10 m from each drain. The distances of these auger holes in vertical direction on collector ditch, 5, 15, 25, 35, 45, and 55m. In Fig. 1, the drain lines and location of auger holes are shown schematically. 24 hrafter rainfall the depth of water table was recorded and the discharge rate of the drains was measured volumetrically. 621

3 Month Table 1. A summary of recorded meteorological parameters Total Max Min.Temp.( Max. Temp. ( Mean rainfall(mm) rainfall(mm) Temp. April May June July Sept Oct Nov Dec Average relative humidity (%) Figure1. schematic location of drain installation and observation wells( shows the location of discharge rate of pipe drains, A,B& C show drain lines and indices show distance from outlet pipe or collector ditch, L and R show the location of auger holes in the left and right drains and indices show the distance of auger holes from each drain ( shows the auger hole in right drain A and to the distance 0.5 m from it in 25 m of collector ditch). Based on the water table measurements, out of 37 recorded rainfall events, only 11 eventscausestheincreasein water table. As a result,the amounts of these discharge rates and water table depths were used for our purposes.in Table 2, the dates of drain discharge and water table depth measurementand total rainfall related to each event was recorded. To determine soil texture,soil sampling was done at different depthsof some observation wells dug for measuring water table depths. Soil hydraulic conductivity was measured by Ernst (direct auger hole) method. For this purpose, 10 observation wells dug for water table monitoring were used.the measurements were made after rainfall or irrigation and in saturated condition of soil.from the below equation, hydraulic conductivity was calculated (Alizadeh, 2004): K =!!" (1) # = $ % % $ % % (2) In the above equations, K is the hydraulic conductivity ( & '() *, r is the radius of observation well(cm), H is the depth of water table (cm),!+, time increment (sec),!y(change in the water table in!t), y mean primary and final water tables recorded (cm), #,, #, hydraulic conductivities of layers 1 and 2, k 12 hydraulic conductivity of the 622

4 soil layer with depth -,.-. Soil textures of 0-150, cm are silty clay and clay, respectively and their hydraulic conductivity were 0.88 and 0.46 /& *'(), respectively. The data obtained in this research were analyzed with t-test, using SAS software. Table 2. Dates of measurements of discharge and water table depths Rain duration(day) Rainfall (mm) Dates of measurment /6/ /8/ /10/ /10/ /10/ /10/ /10/ /11/ /11/ /11/ /12/2011 RESULTSAND DISCUSSION Drain Discharge Discharge rate of drains with and without plastic envelopes (B and A drains respectively), for different dates were drawn in Fig.2.As it can be observed in all dates, the discharge rates of B wereless than discharge rates of A. Assuming the measured dailydischarge is equal to mean discharge rate of the day, then, total daily drainage volume of drains was calculated. Average discharge rate 0 standard error of drain discharge and volume of drainage water are presented in Table 3. Mean discharge rate of drain B in the duration of measurement was * 53/ wich is 12% less than that of drain A ( * 53/ ). Also,there is significant difference between mean discharge rates of drains at 5%confidencelevel and volume of drain A had significant difference with volume of drainage water B. Figure 2. Measured discharge rate from drains in different times Table 3. Mean 0 standard error of drain discharge rate ( * 53/ ) and drainage water volume (& 6 along with probability level of significant difference (P value) calculated by t-test. Volume of drainage water (liter) Discharge rate ( * 53/ Treatment Drain A Drain B P value 623

5 Water table depth The average water table depth of total duration for all observation wells between drains A and C, dug perpendicular to drains, were drawn. Typical curves for observation wells located at25 and 45 m apartfrom collector ditch are shown in Figs. 3 and 4. As it can be observed, the trend of changes of water table depth for both 25 and 45 m distances from collector ditches was quite similar. Amount of measured water table depth in observation well inside trench B was less than corresponding amountsin drains A and C. The water table depth at 0.5 and 1.5 m distances from drain B was also under the effect of plastic cover and was less than observed values in the same distances from drains A andc. The changes of water table depth in 5 m distance from drains were almost similar. In general, the plastic cover had the highest effect on water table depth inside the drain trench and the effect reduced with increase in distance, so that in 5 m distance, the changes in water table depth for 3 drains were similar. The mean water table depth of all observation wells inside trench of drain A, was 9.1 cm more than that in trench drain B. Also, mean watertable depths at 0.5, 1.5 and 5 m distances from drain A were 5.2, 2.9 and 0.5 cm more than corresponding values in drain B. It seems that plastic cover causes lateral movement of water to the drain by preventing of downward flow into drain. This causes a lag time in movement in movement of water around drains into the drains which makes higher water table depth on drain B with respect to drains A and C and its lessdrawdown. Figure3. Water table depth for observation wells dug in 25 m distance from collector ditch. Figure 4. Water table depth for observation wells dug in 45m distance from collector ditch. 624

6 Statistical comparison of water table depths The statistical comparison results of water table depths are presented in tables 4 to 7.Mean observed water table depths in all observation wells inside trench of drain A wasmore than those in drain B and had significant difference at 5% probability level with them (Table 4). According to Table 5, a relative similarity existed for observation wells located at 0.5 m distance from drains A and B. In this case, also, mean water table depths in 0.5 m distance from drain A was always greater than their equivalent values in drain B and had significant difference at 5% level. For observation wells at 1.5 mfrom drains, although mean water table depth was higher in drain A than in drain B, but at 5% probability there was no significant differences between equivalent factors(table 6).According to the Table 7, measured mean water table depth in observation wells with 5 m distance from both drains was almost similar and they had no significant difference at 5% probability level.the decreasing trend of water table depth with increase in distance from outlet of drain pipe can be related to the effect of collector drain and slope of laying pipe drain on the ground. Table 4.Mean 0 standard error of water table depth measured in observation wells of inside drain tranch along with probability level of significance (P value) calculated by t-test Treatments Drain A Drain B P value Table 5. Mean 0 standard error of water table depth measured in observation wells located at 0.5 m from drain, along with probability level of significance (P value) calculated by t-test Treatments Drain A Drain B P value Table 6. Mean 0 standard error of water table depth measured in observation wells located at 1.5 m from drain, along with probability level of significance (P value) calculated by t-test Treatment Drain A Drain B P value Table 7. Mean 0 standard error of water table depth measured in observation wells located at 5 m from drain, along with probability level of significance (P value) calculated by t-test Treatment Drain A Drain B P value Effect of plastic envelope on the clogging of drain envelope By measuring sediment loads of drains at the time of measurement of drain discharge rate, the effect of plastic cover on clogging of drain envelope was determined. To do this, the dry mass of sediment was determined in a specific volume of drainage water and then total sediment load was estimated based on the volume of daily drainage water. In Table 8, the amount of estimated sediment load in drainage water of drains A and B are presented. The sediment load of drain A was always higher than that of drain B. Total sediment in drain A was 74% more than of drain B which shows the considerable role of downward flow to drain in sediment transfer into drain. It seems that plastic cover, by prevention of direct flow from above into the drain and conduction of it to both sides of pipe drain, gives enough time to settle the sediments before entrance into drain.also,less depth of water table around drain B, in comparison to drain A, due to use of plastic cover, causes less hydraulic head to create flow and as a result flow reduction toward drain by which a large amount of sediment can be settleddown before entering the drain trench. 625

7 CONCLUSIONS Plastic cover had the most effect on the water table depth inside the draintrench and by increasing distance from drain, the effect decreased, so that in 5m distance, the water table depth variation was relatively similar for three drains.also the sediment load by drain with plastic cover was higher than that of without plastic cover.based on the results, downward flow of water from upper side of drain into drain has considerable role in sediment transport into drains. Plastic cover prevents direct entrance of water into drain and gives enough time for sediment to settle before entering into pipe drain, hence causing decrease in pipe and drainage envelope clogging. Table 8.Volume of daily drainage water and mass of equivalent sediment from drains A and B. Date of measurement Drain 13/12/ /11/ /11/2011 8/11/ /10/ /10/ /10/2011 5/10/ /9/ /8/ /6/ Drainage water drain A(& Sediment draina(g) Drainage of drain B (& Sediment Drain B(g) REFERENCES Alizadeh A Land drainage, design and planning of drainage systems in agriculture 5thAstanGhodsRazavi pub., 448 pp. Bybordi M Principle of drainage engineering and land reclamation.8 th ed. University of Tehran pub.,in Persian. Dierickx W Electrolytic Analogue study of the effect of Openings Surrounds of Various permeabilities on the performance of field Drainge pipes.rep., 77 National Instittute of Agriculture engineering Merelbeke, Belgium. Kabusi K, Liaghat A, Rahimi H Applicability of rice crust as an envelope in underground drainage. 4 th technological workshop of drainage Karimi V, Yosifian H, Salmani MGh Evaluation of underground drainage system with rice crust in rice lands. 2nd national conference of experience on constraction of water structures and irrigation of drainage networks Ministry of Agriculture and Food, British Columbia Drain filters and envelopes. Drainage factsheet. No. 541,240 pp. National committee on irrigation & Drainage Underground drainage, planning, pp 26 and exploitation. 254 pp. National committee on Irrigation and drainage Materials and pp 26 of underground drainage systems.340 pp. Ramazani Moghaddam JA, Hoshmand A, Naseri Gh, Alizadeh A Evaluation of artificial coverage by physical models for decrease in entering drainage water to rivers. proceedings of 4 th National conference on watershed management and management of soil and water resources: Ritzema HP Performance Assessment of Subsurface Drainage System, case Studies from Egypt and Pakistan. inwageningen, Alterra, The Netherlands, 137 pp. Stuyt LCPM, Dierickx W, Martinez B Materials for subsurface land drainage systems, FAO Irrigation and Drainage Paper, No. 60:183 pp. Stuyt LCPM, Dierickx W Design and performance of materials for subsurface drainage system in agriculture. Agriculture Water Management (86):