Comparing spatial and temporal variability of the system Water Use Efficiency in a Lower Mississippi River watershed

Study Region: The Obion River watershed, Tennessee, is representative of Lower Mississippi River watersheds with loess soils and a fragipan layer. Study Focus: Spatial and temporal heterogeneity of Water Use Efficiency (WUE) and system WUE (sWUE) are examined under different landscapes. Unlike WUE, sWUE considers runoff, better reflecting coupled soil-management-climate effects on an agricultural system. Representative hillslopes grouped in Hydrologic Response Classes (HRCs) were modeled from 1980 to 2050 using the Water Erosion Prediction Project to quantify yields, water budgets, and WUE/ sWUE. HRC-I is flat with clayey soils, low infiltration, and high evapotranspiration. HRC-II has moderate slopes, silty soils, and fragipan layers causing low infiltration and high runoff. HRC-III is similar but with no fragipan producing moderate infiltration and low runoff. New Hydrological Insights: HRC-III experiences 60 % less runoff than the other HRCs due to loess soils with no fragipan. It is the only HRC to store water during the growing season. Clay soils in HRC-I and fragipan layers in HRC-II have less capacity to hold water. sWUE is a more useful index than WUE to describe differences in water holding capacity and ultimately yield across agricultural landscapes. sWUE varies spatially from its correlation with runoff, which has a coefficient of variation (CV) of 71 % across the watershed. Evapotranspiration has a CV 16 %. Temporally, sWUE decreases 10 % by 2050, but its spatial variability reaches 25 %.