P1051 - Analysis of Water Flow under Trickle Irrigation I...

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Analysis of Water Flow under Trickle Irrigation: I. Theory and Numerical Solution F. Lafolie, R. Guennelon, and M. Th. van Genuchten* ABSTRACT Estimating the size of the ponded surface area is a major source of error in modeling water movement under trickle irrigation. After reviewing available approaches and models for simulating water flow under trickle irrigation, we present a new numerical solution of the governing partial differential equations that improves the prediction of the size of the ponded area at the soil surface. A common for- mulation could be used for both Cartesian and axicylindrical flow geometries. The spatial operator was approximated using finite dif- ferences, and time integration was performed with an implicit method. Nonlinearity was treated by means of a fixed point iterative algorithm. The proposed model is applicable to saturated-unsatu- rated flow in layered soils having anisotropic hydraulic properties. Accuracy and stability of the solution are checked and comparisons made with previous numerical solutions for several trickle irrigation and other unsaturated flow problems. I NCREASED IRRIGATION EFFICIENCY, reduced water stress, ease of automation, the ability to apply fer- tilizers and other chemicals with irrigation water, and low economic cost are some of the factors that have contributed to the increased popularity of trickle ir- rigation over the last few decades (Bucks et al., 1982). Trickle irrigation is now applied to a wide variety of crops grown on soils with a broad range in soil texture, soil structure, and hydraulic properties. Notwith- standing its popularity, surprisingly little attention has been paid to estimating soil water distributions during trickle irrigation under realistic field conditions. Lack of understanding of how soil water content distribu- tions are affected by the unsaturated soil hydraulic properties has sometimes resulted in suboptimal man- agement and low water-use efficiency (Hillel, 1985). The shape and total volume of the wetted soil region below a trickle emitter varies widely with irrigation and soil hydraulic parameters. Using the number of drippers, the discharge rate, and the irrigation fre- quency as adjustable parameters, a trickle irrigation F. Lafolie and R. Guennelon, Station de Science du Sol, INRA, Domaine St-Paul, B.P. 91, 84140 Montfavet, France; M.Th. van Genuchten, USDA-ARS, U.S. Salinity Lab., 4500 Glenwood Drive, Riverside, CA 92501. Received 2 May 1988. Corresponding au- thor. Published in Soil Sci. Soc. Am. J. 53:1310-1318 (1989). system should be designed such that the wetted soil volume matches as closely as possible the crop rooting pattern. Soil hydraulic properties usually exhibit consider- able spatial variability at the field scale. An important task in the design of a drip irrigation network is to evaluate how spatial variability affects water content distributions in the field. Particularly useful would be a sensitivity analysis which shows how deterministic soil water contents are affected by variations in rele- vant model parameters. One
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