Critical zone water balance over thirteen years in a semiarid savanna

Authors: Scott, Russell - Russ; Biederman, Joel

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2018
Publication Date: 1/24/2019
Publication URL: https://handle.nal.usda.gov/10113/6471165
Citation: Scott, R.L., Biederman, J.A. 2019. Critical zone water balance over thirteen years in a semiarid savanna. Water Resources Research. 55:574-588. https://doi.org/10.1029/2018WR023477.
DOI: https://doi.org/10.1029/2018WR023477

Interpretive 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 over-riding importance of ET in semiarid watersheds’ water balance and reveal a surprising degree of inter-seasonal water storage.

Technical Abstract: 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.