Monitoring effects of wildfire mitigation treatments on water budget components: a paired basin study in the Santa Fe watershed, New Mexico
Authors: Lewis Amy C. 
Published: April 2023

A paired basin study in the upper Santa Fe River watershed following forest restoration has successfully measured water budget components in a treated and an untreated (control) basin. The paired basin study was established to investigate questions that have arisen with regards to changes in water yield from forest treatments. If forest treatments, for instance, increase the water yield or increase the sustained flow in streams, this could have implications for sensitive ecosystems or downstream water users that require sustained flow. Precipitation, streamflow, soil moisture, and chloride concentrations in precipitation and streamflow were measured to quantify the water budget components. The results from nine years of data collection and analysis show a high degree of confidence with respect to measuring the water budget components based on the mass balance of water and chloride.

The cycle of chloride entering and exiting each basin is examined over five integration periods. The total inflow of chloride from precipitation is assumed to be equal to the outflow of chloride in streamflow and recharge over each integration period. Volume-weighted chloride concentration in precipitation ranges from 0.18 to 0.24 mg/L for the five integration periods. The volume-weighted chloride concentration in streamflow for the same periods ranges from 2.2 to 3.2 mg/L in the treated basin and 0.9 to 1.4 mg/L in the control basin. The difference in chloride concentrations between the two basins was observed prior to forest treatments. Based on the ratio of chloride concentration in precipitation to the chloride concentration in streamflow, outflow of water due to evapotranspiration (ET) is estimated to be about 90 to 94% of precipitation in the treated basin and 77 to 86% in the control basin, within the same range as observed prior to forest treatments. The higher ET in the treated basin both before and after forest treatments may be due to the much greater area of western slope in the treated basin that receives warm afternoon sun and the greater area of rock cover in the control basin. In this investigation, changes in the ratio of water budget components in the control as compared to the treated were the focus of this investigation. While the pre-treatment data before 2004 is limited, treatments will continue to occur in order to achieve a forest structure that is more resilient to wildfire.

Estimates of recharge, based on the chloride mass balance, range from 1.7 to 7.3% of precipitation in the treated basin and 1.1 to 13% in the control basin. While ET appears to decrease over time following forest treatments in the treated stream relative to the control basin (based on the chloride ratio), changes in streamflow and recharge are only observable during periods when winter precipitation represents a greater proportion of the annual precipitation. The relatively dry period of this nine-year investigation may have contributed to the lack of overall discernable differences in streamflow and recharge. Forest canopy reduction and increased ground cover appears to have reduced the peak hydrograph in response to intense storm events in the treated basin.