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Critical Zone Water Balance Over 13 Years in a Semiarid Savanna

Scott, Russell, Biederman, Joel
Water resources research 2019 v.55 no.1 pp. 574-588
arid lands, climate, drainage, evapotranspiration, groundwater recharge, growing season, land use, rain, rhizosphere, runoff, savannas, soil water, spring, summer, uncertainty, water storage, winter
Quantifying how much and when precipitation (P) becomes runoff (R), evapotranspiration (ET), and drainage (D) in the critical zone is key to understanding how climate and land use impact hydrology. We quantify water balance dynamics of a semiarid savanna with a summer/winter rainfall pattern with 13 years of water fluxes and soil moisture. We find multiyear P is partitioned 96% to ET and 7% to R, while D is negligible. While weather regulates ET over diurnal timescales, soil water inputs control seasonal to annual ET amounts. Here we establish that seasonally-integrated ET is a better metric of ecosystem-available water than time-averaged soil moisture or precipitation, some of which is lost to runoff. Contrary to expectations, we find significant, episodic carryover of soil moisture from one growing season to the next. Abundant late-summer P can supply ET in the subsequent spring, even after multi-month dry periods. However, over an annual cycle nearly all soil moisture inputs are used by ET. Likewise, drainage beyond the monitored root zone occurs within a season, but this is counteracted by subsequent ET extraction of deep moisture over the year. Thus, very little groundwater recharge (net drainage) occurs at this site, though there is considerable uncertainty in estimation of this small flux as the residual of much larger ones. These comprehensive, long-term measurements support expectations about the over-riding importance of ET in the dryland critical zone water balance and reveal a surprising degree of inter-seasonal water storage.