Flood Irrigation of a Cracked Soil

An understanding of water flow into cracking, irrigated soils is necessary in order to address problems of plant water stress, inefficient water application, lack of aeration, and salt accumulation in the soil due to inadequate leaching. The objectives of this study were to investigate water infiltration into a cracked clay soil during flood irrigation, and to observe the differences in infiltration and cracking patterns between fallow soil following wheat (Triticum turgidum L. 'Yecoro Rojo') and soil under alfalfa (Medicago saliva L. 'CUF 101'). A large weighing lysimeter was used to measure infiltration and evaporation. The infiltration was analyzed as consisting of three phases of crack filling, sorption, and transmission. The infiltration curves were found to be similar for all irrigations, with a larger percentage of the total infiltrated water initially entering the cracks of the drier alfalfa soil (74%) than the wheat-cropped soil (63%). Final infiltration rates were 0.4 and 0.6 mm h' for the alfalfa and wheat irrigations, respectively. Evaporation was shown to be a large component of water loss during the later stages of irrigation, sometimes exceeding the infiltration rate. Cracking patterns could be observed because of the presence of foam, which consists of organic acids picked up by water rising in the cracks. The wheat-cropped fallow soil had more numerous cracks than alfalfa-cropped soil, which was attributed to its fiberous root system, which is weaker than the alfalfa taproot system. The difference in cracking patterns between the two crops has implications for water flow. A UNDERSTANDING of the nature of infiltration in cracking soil under flood irrigation is necessary in order to address the problems of poor permeability in irrigated, arid-zone soils. Poor permeability during infiltration and redistribution may have severe economic consequences for irrigated agriculture when it results in inadequate water storage, inefficient and nonuniform water application, lack of aeration, or excess salinity (Oster and Singer, 1984). Many of the poorly permeable soils, such as those in California's Imperial Valley, are alluvial soils (fluvic Entisols) with smectite mineralogy (Perrier et al., 1974). Seventy percent of the irrigated soils there are finetextured with relatively low infiltration rates (IRs) (Zimmerman et al., 1980). Flood irrigation is widely used because of several management considerations, including the presence of level fields, and the availability of relatively inexpensive irrigation water from the Colorado River. Silt makes up a large component of these soils, which causes them to shrink at rates that are less than the unitary shrinkage rate (Mitchell and van Genuchten, 1992), a phenomenon not uncommon for cracking soils (Jayawardane and Greacen, 1987). Water flow in cracking soil consists of a dichotomy of rapid flow in cracks and very slow flow in the A.R. Mitchell, Oregon State Univ., Central Oregon Agricultural Research Center, 850 NW Dogwood Lane, Madras, OR 97741; and M.Th. van Genuchten, USDA-ARS, U.S. Salinity Lab., 4500 Glenwood Dr., Riverside, CA 92501. Contribution of the USDAARS, U.S. Salinity Lab. Received 5 Apr. 1991. 'Corresponding author. Published in Soil Sci. Soc. Am. J. 57:490-497 (1993) swelling soil. Water can be conducted rapidly in the cracks of an otherwise impermeable soil (Yong and Warkentin, 1975). For example, agronomic practices that disturb the soil surface, such as intensive grazing, having been shown to have no negative effect on water infiltration because of the flow of water into cracks (Mitchell et al., 1991). As suggested by Swartz (1966), the total infiltrable volume of water may be more indicative of the irrigation process than the IR of the soil, especially if the final IR is very low. Soils in the Imperial Valley of California have been documented to have low final IRs (Perrier et al., 1974), as do other irrigated swelling soils, including Vertisols in India (Shanware and Gawande, 1989) and soils in Queensland, Australia (Swartz, 1966). The challenge of irrigation management of cracking soils is to take advantage of the rapid water intake rate of a dry, cracked soil, while keeping plant water stress at a minimum. Infiltration equations for nonmacroporous soil need to be modified for cracking soil. Water flow in nonmacroporous soil is theorized as consisting of sorptive flow and gravitational flow, and corresponding terms may be found in versions of the classic Richards' equation, where a diffusivity-like term determines sorptive flow and a conductivity term described gravitational flow. Likewise, empirical infiltration equations usually contain a sorption term that parameterizes water sorption and a gravity-driven term that is associated with the saturated hydraulic conductivity (e.g. Philip, 1957; Collis-George, 1977). For soils dominated by macropores, an additional term is needed in the equation to describe the water intake into the cracks (Collis-George, 1977). During the crack-filling (CF) phase of flood irrigation, the IR is equal to the water application rate. The initial intake of water into soil cracks prior to ponding has been estimated by Bridge and Ross (1984) to be approximately two-thirds of the total intake. Unfortunately, there is little published field data to support the two-thirds rule. One reason may be the difficulty of measuring the amount of water added during CF; this measurement is not amenable to the standard cylinder infiltrometer (Bouwer, 1986) measurement of ponded irrigation because of the lateral loss of water from the measurement area through cracks. Attempts to use extracted cores to describe the hydraulic properties have failed since they are not representative of the cracks (Ritchie et al., 1972). Studies of rainfall infiltration into cracked clay have produced valuable information about cracking morphology (Bouma, 1984), but are inadequate to describe water intake during the flood irrigation process. Under rainfall conditions, only runoff water will flow into the crack; whereas during flood irrigation the water flows first into the large cracks. Sprinkler irrigation may result in the closure of cracks near the soil surface, which would limit water intake compared with flood irrigation. For these reasons, sprinkler infiltrometer Abbreviations: IR, infiltration rate; CF, crack filling; ER, evaporation rate.