Evaluating the complimentary relationship of evapotranspiration in an arid shrublands

Accurate estimates of evapotranspiration and its components are essential for quantifying the water and energy fluxes and water resources management in arid regions. To this end, daily actual evapotranspiration (ET_a), pan evaporation, and concurrent microclimate from an arid shrublands were measured over two growing seasons (2014–2015) to determine water budgets and to test the validity of the complementary relationship (CR) at this temporal scale. The average ET_a is 229.32 ± 45.86 mm during two growing seasons, while canopy transpiration, soil evaporation, and interception accounted for 68.1 ± 16.5%, 29.1 ± 2.5% and 2.8 ± 0.6%, respectively. Actual evapotranspiration and Penman potential evapotranspiration, or pan evaporation exhibit complementary behavior, where the complementary relationship is asymmetric. Daily ET_a rates are significantly overestimated by the symmetric Advection-Aridity (AA) model. Employing the modified AA model, where parameters are calibrated locally and wet environment evapotranspiration is evaluated at wet environment air temperature as opposed to the measured air temperature, the prediction accuracy of ET_a is dramatically improved. With calibrated parameters, the E601B sunken pan can satisfactorily describe the dynamics of daily ET_a, while the D20 aboveground pan underestimates it to some extent. Moreover, the modified AA model is able to capture the dynamics of groundwater usage by vegetation during dry summer. These findings gain our new knowledge on the capability of CR theory to resolve special issue occurred in phreatophytic shrublands, and can also provide beneficial reference to water resource and eco-environment management in arid regions.