Evapotranspiration and Runoff Patterns Across California's Sierra Nevada

Abstract

Spatially resolved annual evapotranspiration was calculated across the 14 main river basins draining into California's Central Valley, USA, using a statistical model that combined satellite greenness, gridded precipitation, and flux-tower measurements. Annual evapotranspiration across the study area averaged 529 mm. Average basin-scale annual precipitation minus evapotranspiration was in good agreement with annual runoff, with deviations in wet and dry years suggesting withdrawal or recharge of subsurface water storage. Evapotranspiration peaked at lower elevations in the colder, northern basins, and at higher elevations in the southern high-Sierra basins, closely tracking the 12.3°C mean temperature isocline. Precipitation and evapotranspiration are closely balanced across much of the study region, and small shifts in either will cause disproportionate changes in water storage and runoff. The majority of runoff was generated below the rain-snow transition in northern basins, and originated in snow-dominated elevations in the southern basins. Climate warming that increases growing season length will increase evapotranspiration and reduce runoff across all elevations in the north, but only at higher elevations in the south. Feedback mechanisms in these steep mountain basins, plus over-year subsurface storage, with their steep precipitation and temperature gradients, provide important buffering of the water balance to change. Leave-one-out cross validation revealed that the statistical model for annual evapotranspiration is sensitive to the number and distribution of measurement sites, implying that additional strategically located flux towers would improve evapotranspiration predictions. Leave-one-out with individual years was less sensitive, implying that longer records are less important. This statistical top-down modeling of evapotranspiration provides an important complement to constraining water-balance measurements with gridded precipitation and unimpaired runoff, with applications such as quantifying water balance following forest die-off, management or wildfire.

Water balance for gaged watersheds in the Central Sierra Nevada, California and Nevada, United States

“…Overall, ET measurements were robust across the range of elevations and mountain-vegetation types represented in this study. That is, the good basin-scale water balances previously observed in larger-basin water-balance assessments in the Sierra Nevada (Roche et al, 2020;Rungee et al, 2021) are also apparent in the smaller headwater basins across mid to higher elevations that were the subject of the current analysis. After accounting for apparent diversions (adjusted Q) and underestimated precipitation (adjusted P) we found very good to excellent water balances across 33 of the 48 headwater watersheds.…”

“…While ET reductions of 5-10% may be appropriate in some cases, uncertainty in annual Q and P is apparently larger, and thus adjustments to those components are more appropriate. We estimate that while uncertainty in ET from a given flux tower used to develop the gridded ET data could be as much as 20%, the uncertainty should be random from tower to tower, and to a lesser extent year to year, so the overall uncertainty should be less for the full dataset (Rungee et al, 2021). NDVI values, resolved in our study at 30-m from Landsat, are correlated with ET in the Sierra Nevada (Goulden et al, 2012;Goulden and Bales, 2014).…”

“…Our P-values, from PRISM, are an 800-m gridded product, and fail to capture the multi-scale variability in higher-elevation precipitationsnow accumulation -observed by Lidar (Zheng et al, 2016), snow pillows (Kirchner et al, 2014), or snow courses (Rice and Bales, 2010). We previously assessed that the total uncertainty in precipitation may be near or less than the reported west-wide potential annual interpolation error of ±98 mm for PRISM in rain-dominated areas with more data, and upwards of 50% or more in snow-dominated, open areas (Rungee et al, 2021).…”

Mechanisms Controlling Carbon Sinks in Semi‐Arid Mountain Ecosystems

“…Model performance in reproducing the observed gridded ET for 2001–2017 (Rungee et al., 2021) across the Sierra Nevada was good (Figure S13 in Supporting Information S1), with the R 2 value of 0.76, and root mean squared error (RMSE) 106 mm. The model somewhat overestimated ET at the lower end, but underestimated ET at the higher end.…”

“…The sites extend from 129 to 2700 m in elevation and represent the major forest types (Figure 1). In addition to these point measurements, we also used a spatially gridded ET data set (30 × 30 m) (Rungee et al., 2021) for a secondary evaluation of ET simulated by LPJ‐GUESS. These gridded ET products were derived using a multivariate calibration equation that relates observed ET at flux towers with Landsat‐derived Normalized Difference Vegetation Index (NDVI) and annual precipitation.…”