Patterns and associations between dominant crop productions and water quality in an irrigated watershed

Irrigation consumes the largest share of freshwater resources, but is a necessary practice to boost agricultural output to meet increasing global demand for food and fiber. Irrigation not only impacts water quantity but can also degrade water quality. Research efforts have explored various aspects of irrigation efficiency and irrigated crop productivity, but few studies have examined how different crops collectively modulate water utilization and water quality at the watershed scale. The objective of this study was to determine how the fractions of evapotranspiration (fET) water ascribed to major crops impact water quantity and quality in irrigation return flow. In this study, long-term water quantity and quality monitoring data, collected as part of the Conservation Effects Assessment Project (CEAP), combined with crop and evapotranspiration (ET) modeling products, were used to build relationships between water quantity and quality metrics and fET associated with major crops during the first 15 years of the CEAP Twin Falls irrigation project. Results suggest that subwatershed size and subsurface flow contribution in regional drainage tunnels influenced the observed hydrologic patterns and led to two distinct groups. Subwatersheds in group 1 were large, typically included subsurface drain tunnels, and had high return flow volumes and low sediment concentration, while those in group 2 were smaller in size, had low return flow volumes, and high sediment concentration. Multiple linear regression analyses showed that spring and summer irrigation return flow volumes normalized by subwatershed area increased as a function of fET of potato (Solanum tuberosum) in group 1 (regression coefficients [coef.] = 4.42 in spring and 1.54 in summer) but were inversely associated with small grains in the fall (coef. = −1.67 and −0.60 in groups 1 and 2). Spring sediment concentration had negative regression coefficients with fET of sugar beet (Beta vulgaris) (coef. = −911.00) and alfalfa (Medicago sativa) + pasture crops (coef. = −424.85) in group 2. When statistically significant, a negative association was found between phosphorus (P) load per return flow volume and fET of alfalfa + pasture (coef. = −0.68 to −1.07), corn (Zea mays) (coef. = −0.64 to −0.89), dry beans (Phaseolus vulgaris) (coef. = −1.25 to −1.87), and sugar beet (coef. = −1.54 to −2.83) across aggregation periods and subwatershed groups. Nitrate (NO₃-N) load per return flow volume was negatively associated with potato and corn fET in group 1 especially during the spring (coef. = −31.13 for potato and −9.60 for corn) and fall (coef. = −14.54 for potato and −4.43 for corn) months but positively associated with dry beans (coef. = 4.87) over the irrigation season. While direct cause and effect were not established with this analysis, results from this study provide valuable information about various crop production systems that may impact observed hydrologic responses.