Critical Zone Water Balance Over 13 Years in a Semiarid Savanna

Quantifying how much and when precipitation (P) becomes runoff (R), evapotranspiration (ET), and drainage from the root zone (D) is key to understanding how climate and land use impact hydrology of the critical zone. We quantify water balance dynamics of a semiarid savanna with a summer/winter rainfall pattern with 13years of water fluxes and soil moisture. We find multiyear P is partitioned 96% to ET and 7% to R, while D (-3%) is negligible when considering measurement uncertainty. While weather regulates ET over diurnal time scales, soil water inputs control seasonal to annual ET amounts. Seasonal water availability, estimated by soil moisture inputs, is more closely tracked by ET rather than time-averaged soil moisture or P. Surprisingly, we find significant, episodic carryover of soil moisture from the summer to spring growing season. Abundant late-summer P can supply ET in the subsequent spring, even after multimonth dry periods. However, over an annual cycle beginning in early summer, nearly all soil moisture is used by ET. Likewise, D beyond the monitored root zone, assisted by downward hydraulic distribution in plant roots, occurs within a season, but this is counteracted by subsequent ET extraction of deep moisture over the year. Thus, negligible long-term D occurs, 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 overriding importance of ET in the dryland critical zone water balance and reveal an unexpected degree of interseasonal water storage. Plain Language Summary One of the most enduring and important questions for hydrology is how water input in the form of precipitation is partitioned among evapotranspiration, runoff, groundwater recharge, and storage of moisture in the soil. We quantified how precipitation was partitioned at a semiarid savanna site in Arizona, USA, with 13 years of data. We found that almost all of the precipitation goes into evapotranspiration with only a small of runoff and negligible recharge. Contrary to expectations, we saw significant, episodic carryover of soil moisture from the summer/fall growing season to the subsequent springtime when the plants awake from winter dormancy and extract the stored moisture. These comprehensive, long-term measurements support expectations about the overriding importance of ET in semiarid watersheds' water balance and reveal a surprising degree of interseasonal water storage.