Use of a Lumped Seasonal Agricultural Flow in an Inverse End-Member Mixing Analysis to Understand Transport of Water and Solutes in Irrigated Catchments

Irrigation water can be a major component of the hydrologic budgets of agricultural catchments in arid regions and can play a dominant role in the transport of solutes in those catchments. Hydrologic, chemical, isotopic, age-dating, and mineralogical data were collected over a full year from the irrigation water supply canal, ground water, overland flow, streambed pore water, and the catchment outflow of a small (6 km2) agricultural catchment in central Washington. During the irrigation season, outflow from the catchment was 2 to 3 times greater than during winter. The water chemistry at the outflow also varied substantially between the irrigation season and winter. Based on these observations, a simple, end-member mixing analysis (EMMA) was formulated for the catchment that expressed the outflow as the sum of ground-water base flow plus seasonal agricultural flow (SAF). Conceptually, SAF is a hypothetical composite of excess irrigation water and includes system spill, tailwater, overland flow, and applied irrigation water that is not incorporated into the ground-water system, but infiltrates and travels via shallow, short subsurface flow paths (e.g., tile drains) to the surface water system. Because winter catchment outflow was virtually all ground-water base flow, summer base flow was estimated by adjusting winter catchment outflow measurements to reflect seasonal changes in the hydraulic head gradient between ground-water and the stream. Winter chemistry data from the catchment outflow were used to approximate summer base-flow chemistry. Using measured summer catchment discharge and chemical characteristics (nutrients and major ions) and the estimates for summer base-flow characteristics, the discharge and chemical characteristics of SAF were calculated using the inverse formulation of the EMMA. Comparing and contrasting the chemical characteristics of SAF with canal water provided insight into the composite effects of agriculture on the catchment's water chemistry. In addition, because ground-water base flow in the catchment can virtually all be attributed to irrigation over the past century, differences between SAF and base flow provided information on water chemistry changes resulting from transport through the subsurface and(or) differences in agricultural practices over time. Finally, comparing characteristics of SAF to measured characteristics of water from a single field outflow provided insight into how well the net effect of processes at the field scale approximated the net effect of processes at the catchment scale. This simple inverse EMMA was a useful tool for testing and refining the conceptual model for this irrigated catchment.