Quantifying water stress and productive vs non-productive water use in the critical zone of dryland agricultural areas with alternating surface and groundwater irrigation: lessons learned from a combined remote sensing and groundwater data analysis.

In drylands, where high temperatures and limited water availability constrain farming activities, irrigation planning is essential for the efficient water use and long-term sustainability of agriculture. However, in spite of dwindling water supplies and the looming threat of more frequent and intense droughts, low efficient irrigation (e.g. flooding) continues to be the most widely used method for water delivery into agricultural parcels. Transitioning into a more sustainable agriculture requires knowledge of the period of productive vs non-productive crop water use during the agricultural cycle to maximize the former and minimize the latter. In this work, we present a method to estimate Evapotranspiration and its partition into Evaporation and Transpiration as a function of the phenological evolution of the crop, its areal coverage, the prevailing meteorological conditions in the area and the energy balance of the parcel. We verify our ET estimates with independently derived ET using a modified White method and sub-hourly groundwater data of the shallow aquifer from which we also quantify crop stress. Our study area is a Pecan Orchard located in the Lower Rio Grande Valley in Tornillo, Texas. Using images derived from Landsat 8 and the METRIC algorithm for the period March-December during the 2018 agricultural cycle, the crop coefficient (Kc) and the actual evapotranspiration (ETa) were obtained. Daily ETa values between the 16-day Landsat image intervals, were estimated by interpolating Kc and daily reference Evapotranspiration (ET0) derived from gridded weather data using Penman-Monteith. ET partition was achieved by separating Transpiration estimates obtained from daily ET0 and the changing basal crop coefficient (Kcb) calculated from Sentinel 2 through vegetation indices and adjusting by the vegetation fraction derived from pixel unmixing techniques, and the daily ETa. Our results showed the period of greatest water demand coincides with both, the period of maximum Transpiration and the period of most significant crop stress that is not related to moisture supply. Total ETa during the cycle was estimated in 1,334 mm from which 927 mm (~69.5%) corresponded to transpiration (productive water). Using these results, we determine the potential for the shallow groundwater recharge by means of return flow.