Long‐Term Variability of Soil Moisture in the Southern Sierra: Measurement and Prediction

Abstract

Using 6 yr (Water Year [WY] 2009-WY 2014) of hourly in situ measurements from a spatially distributed water-balance cluster, we quantified the long-term accuracy of an algorithm used to predict spatial patterns of depth-integrated soil-water storage within the rain-snow transition zone of the southern Sierra Nevada. The algorithm-the multivariate, non-parametric regression-tree estimator Random Forest-was used to predict soil-water storage based on a combination of attributes at each instrument cluster (soil texture, topographic wetness index, elevation, northness, and canopy cover). Out-of-bag R 2 (similar to cross-validation for Random Forest) was used to quantify the accuracy of the estimator for unobserved data. Accuracy was consistently high during the wet-up, snow-cover, and early recession periods of average and wet years. The accuracy declined at the end of a 3-yr dry period, and the relative rank of the independent variables in the model shifted. Soil texture was the highest-ranked independent variable across all years, followed by elevation and northness. Topographic wetness increased in importance during dry periods. Northness exhibited high importance during the wet-up and early recession periods of most water years. During dry years, the importance of elevation declined. In dry years, notable differences in soil-water storage at each depth include lower-than-average storage in the deeper regolith at the beginning of the water year and lower storage in near-surface layers during the winter resulting from transient snow cover.

Cosmogenic Isotopes Unravel the Hydrochronology and Water Storage Dynamics of the Southern Sierra Critical Zone

“…A first approximation of unsaturated zone storage in this critical zone is the annual storage change. Unsaturated storage in the top 1 m varies from 100 to 300 mm annually (Oroza et al, ). Annual storage changes averaged over the entire watershed are typically on the order of 600 mm.…”

“…Based on a storage capacity of 200‐mm water per meter depth, 3 m of unsaturated zone are required—on average over the entire catchment—to store the annual storage change of 600 mm (Figure S2.3). At the lowest storage conditions, 100 mm of water remain in the top 1 m of the unsaturated zone (Oroza et al, ). Extrapolating this residual water content to a 3‐m unsaturated zone leaves a residual of at least 300 mm of unsaturated storage.…”

Spatially distributed water-balance and meteorological data from the rain–snow transition, southern Sierra Nevada, California

“…The site has a Mediterranean climate, with cool, wet winters and dry summers that last from approximately May through October. Precipitation falls as a mix of rain and snow, and precipitation transitions from majority rainfall to majority snow typically at about 2000 m in elevation (Bales et al, 2011;Safeeq and Hunsaker, 2016). The upper part of the Providence Creek site lies at about the 50 % rain versus snow elevation.…”

“…Manual measurements for instrument verification were made at twice-monthly visits unless delayed by weather. Precipitation at the weighing gauge was verified against measurement records from the snow pillow and nearby weather stations (Table 2; further information about precipitation data assurance is in Safeeq and Hunsaker, 2016). Table 3).…”

“…The 10 and 15 min data have been averaged to daily values shown here. large (61 cm for P301 and P303; 30.5 cm for P304) and one small (7.6 cm) custom-made fiberglass Parshall flume designed by the FS hydrologist (Moore Sailboats, Watsonville, CA, USA) to capture the range of flows while a 120 • V-notch weir is used at P300 (Safeeq and Hunsaker, 2016). An ISCO 730 air bubbler (Teledyne Isco, Lincoln, NE, USA) is the primary stage-measurement device.…”