RESULTS AND DISCUSSTON. Time, rnin. (Q)

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1 rrigation SOL WATER NFLTRATlON AND REDSTRBUTON UNDER FURROW AND SPRNKLER RRGATON G. Uehara, T. C. Juang and M. sobe Hawaii Agricultural Experiment Station and the Hawaiian Sugar Planters' Association Experiment Station Honolulu, Hawaii ABSTRACT Water was applied to laboratory,?oil columns to simulate furrow or flood and sprinkler irrigation on a strongly aggregated oxisol. Furrow or flood irrigation was simulated by maintaining a 1 cm head of water over the soil surface during infiltration. Sprinkler irrigation was simulated by spraying water on the soil surface at a constant rate. At cessation of infiltration the wetting front under the flooded surface was shallower than the wetting front in the column receiving water as a spray. During redistribution, the wetting front under the flooded surface overtook the one under simulated sprinkler irrigation. This unusual characteristic of water movement in aggregated oxisols was attributed to the dependence of their water content-tension relation on water application rates. NTRODUCTON n Hawaii a sugarcane field or crop can be ~ategorized as irrigated or non-irrigated. f it is non-irrigated the field receives sufficient rainfall to sustain profitable production. f it is an irrigated field, water is applied in the furrow or from a variety of overhead sprinkler systems. n an effort to conserve the water the industry has been experimenting with sprinkler irrigation systems. This report summarizes laboratory work conducted to study soil-water infiltration and redistribution under furrow and sprinkler type irrigation in aggregated oxisols. MATERAL AND METHODS The soil used in this study was the Molokai series classified as a Typic Torrox in the new US Comprehensive System of Classification. As the name implies this soil occurs in the semiarid region of the state. When irrigated, it is an extremely productive soil. Soil columns 40 cm long with an inside diameter of 6.5 cm were packed with soil to a density of approximately 1.1 gm/cm3. The initial water content was 0.05 gm/cm3. Water was added to the surface at rates of 0.43, 0.84, 1.1 and 2.2 in./hour in the form of a spray to simulate sprinkler irrigation. Water was also added to the column to simulate flood irrigation. n this case a -cm head was maintained over the soil surface during infiltration. Water

2 s (J, G. UEHARA, T. C. JUANG, M. SOBE 895 was also applied at a constant rate of 6.3 in./hour. Ponding occurred in this case after 4 minutes of water application. During infiltration cumulative water intake and advance of the wetting front were recorded as a function of time. After cessation of infiltration advance of the wetting front was recorded. RESULTS AND DSCUSSTON Water applied under flood irrigation can be approximately simulated in the laboratory by maintaining a constant positive head over the soil surface. Under these conditions the intake rate is very rapid at early times and drops off at longer times until a steady rate is achieved. The upper curve in Fig. la Time, rnin. (Q) Time, (b) min, Fig. 1. Cumulative water intake as a fullctioll of time under flooded or partially flooded (a) and sprinkler (b) irrigation. Values refer to water application rates in inches per hour. illustrates a typical cumulative water intake vs time relationship. The time derivative of such a curve, plotted as a function of time, provides the infiltration rate. When water is applied under the sprinkler system, the rate of water application is constant, and, thus, the relation between cumulative intak,e vs time is a straight line as long as ponding does not occur. Ponding does not occur if the water application rate does not exceed the infiltration rate of the soil. The lower curve of. Fig. la describes water added to the soil at a constant rate of 6.3 in./hour. Slight ponding occurred 4 minutes after the beginning of water application, showing that the steady state infiltration rate for this soil was just about 6.3 in./hour. Under heavy application rates, water moves downward more rapidly through a soil profile than when water is applied slowly. Fig. 2a and 2b show differences in advance of wetting fronts for different rates of water application. Note that in Fig. 2a the rate of advance decreases with time. This is characteristic of systems in which ponding occurs. When no ponding occurs, it is possible to

3 Time, min. Time, min. (a) (b) 1 Fig. 2. Advance of wetting front as a function of time under flooded or partially flooded (a) and sprinkler (b) irrigation. Values refer to water application rates in inches per hour. 1 add water to a soil at a constant rate. Under these circumstances the relation between advance of the wetting front and time is a straight line. When water is applied at different rates, the water content behind the wetted front is higher in the higher application rates. This is implied in the data plotted in Fig. 3. For equal cumulative water intake, say for example 6 cm, i method of water application: c-c-9 flooded - e d g partially flooded 0.83 in/hr 0.43 inlhr 0Vl" ~ t l l t, Advance of wetting Front, cm Fig. 3. Relations between cumulative water intake and advance of wetting front for 4 different. rates of water application.

4 -, C.. UEHARA, T. C. JUANG, M. SOBE 897 the wetted front has moved to a greater depth in the low application rates. This means that an equal amount of water is distributed over a greater depth when water is applied slowly. ( When an equal amount of water is added to 2 columns containing the 6, same soil, adjusted to identical initial conditions, the rate of advance of the wetting front is greater in the 1 receiving water at higher application rates, not only during infiltration but during post infiltration redistribution. The results in Fig. 4 were obtained by adding 3 in. of water at 2 different rates to identical Time, min. Fig. 4. Advan-e of wetting front as a function of time during and after infiltration under partially flooded and sprinkler irrigation. Arrows point to position of wetting front at cessatioll of infiltration. Three inches of water were added ill each case. soil columns. At cessation of infiltration the wettings fronts were at 17 and 18.2 cm for the high and low application rates, respectively. Fifty minutes after cessation of infiltration the'wetting front in the high rate had moved to 22.5 cm, but the front of the lower rate virtually stopped at 20 cm. This differs from the results obtained by Bresler, Kemper and Hanks (1969) and Hanks, Klute and Bresler (1969). These authors found a shallower wetted zone in the high application rate even after long times after cessation of infiltration. Their results can be readily explained on the basis of the hysteretic nature of the water content-tension curve. n effect a soil which is wetted to a higher water content will release water along a desorption curve which wil; always be higher in water content at a given tension than a similar soil wetted to a lower water content. So long as hysteresis in the moisture characteristic curve is not appreciably altered by the rate at which a soil is drained or wetted, the results obtained by Bresler et al. are not unexpected.

5 898 RRGATON The soil used in this study consists of large, stable aggregates (Sharma and Uehara, 1968). When it is subjected to a heavy rate of water application, water moves rapidly downward through the large interaggregate pores, often isolating pores within the aggregate. On the other hand water added slowly to this soil tends to move slowly through the finer interaggregate pores, providing sufficient time for the fine interaggregate pores to sorb water. n short, even though both columns were wetter from the same initial water content, one must assume that the adsorption water content-tension curve differed for the 2 wetting rates. This is because for the same water content during wetting, water occupies different pore size classes for different water application rates. The practical consequence of this is that water which is added slowly and which, therefore, occupies the fine pores,is held more tenaciously by the soil than an equal amount of water added quickly and distributed in larger conducting pores. Davidson, Nielson and Biggar (1966) have described situations in which the water content at a specific tension depended upon the applied pressure increment or redistribution rate. They found that when tension was changed from 100 cm to 4 cm tension (adsorption) in 4 equal equilibrium steps, a higher water content was achieved than when the tension change was made in one step. For desorption, 1 pressure step produced a lower water content than when the same tension change was made in 4 steps. Differences in results reported llere and results obtained by earlier workers have been mainly caused in this instance by subjecting a soil with a wide pore size distributions with extremely different water application rates. The results of this study may be useful in the choosing and designing of irrigation systems for aggregated tropical soils. REFERENCES Bresler, E., W. D. Kemper, and R. J. Hanks nfiltration redistribution, and subsequent evaporation of water from soil as affected by wetting rate and hysteresis. Soil Sci. Soc. Amer. Proc., 33: Davidson, J. M., D. R. Nielsen, and J. W. Biggar The dependence of soil water uptake and release upon the applied pressure increment. Soil Sci. Soc. Amer. Proc., 30: Hanks, R. J., A. Clute, and E. Bresler A numeric method for estimating infiltration, redistribution, drainage, and evaporation of water from soil. Water Resources Res., 5: Sharma, M. L., and G. Uehara nfluence of soil structure on water movement ill low humic latosols. Soil Sci. Soc. Amer. Proc., 32: