Determination of infiltration characteristics by volume balance for border check irrigation

Abstract

The infiltration characteristics of a cracking clay soil were predicted by a regression approach to the volume balance technique utilising automatic data gathering techniques. The study was conducted at Deniliquin, N.S.W. on a clay loam soil during the irrigation of millet and triticale and the pre-irrigation for a wheat crop. The analysis technique developed enables the fitting of any form of infiltration function by least squares regression. The parameters of three common infiltration equations could be predicted accurately from measured field data, and the best fitting equation identified, however, the parameters of the fitted equations were extremely sensitive to errors in measuring field data (inflow and surface storage). Accurately rated hydraulic water control structures are the preferred method of collecting inflow data, although syphons are suitable if entry and exit conditions can be controlled. In this case, large variations were measured in the rate of discharge of syphons operating under identical head conditions. The average depth of water flowing over an irrigation border was related to the water depth measured at the head of the border, the rate of inflow to the border and the slope of the border. This model of surface storage can be improved by the inclusion of further parameters (e.g. biomass density) not measured in this experiment. Measurement of depth of flow at various points within an irrigation border should be made to determine with confidence the average depth of flow and surface storage of irrigation water. The volume balance technique assumes that water infiltrates only into the soil which is covered by the advancing irrigation water. This assumption holds for soils in the "fully irrigated" state where soil moisture levels are high enough to prevent extensive cracks, it is invalid in dry soils where extensive shrinkage cracks develop. Under these conditions the form of the infiltration characteristic is wrongly predicted leading to an overestimate in the amount of infiltration. Rates of irrigation advance approached a linear form and the rate of advance was more strongly influenced by inflow rates than by border slope or the stage of crop growth. Further experimental work with accurately measured rates of inflow is needed to define better the relationships between inflow rate and rate of advance. © 1993.