An empirical model of the relationship between irrigation and the volume of water collected in subsurface drains

Abstract

Irrigation and drainage data from a water district in San Joaquin Valley, California, are examined to determine the potential for reducing drain water volumes through improved farm-level water management. A regression model of the relationship between irrigation and drain water is estimated. Explanatory variables include applied irrigation water, soil characteristics, cropping patterns, and the location of drainage systems in the district. The responsiveness of drain water volumes to changes in farm-level irrigations and the influence of lateral subsurface flows from higher elevations are examined. The proportion of total collected drain water which is due to lateral subsurface flows is estimated to be 27% in 1988. Farm-level activities account directly for an estimated 64% of total drain water in this year, with the remainder attributed to rainfall and seepage from the main surface drain in the district. The estimated proportional responsiveness of collected drain water volumes to changes in irrigations ranges from 0.78 for drainage systems with significant contribution from lateral flows to 1.78 for drainage systems where these are not significant. It is estimated that a 10% across-the-district reduction in applied water will result in a 15% reduction in the volume of drain water collected.