Recent Patterns in Climate, Vegetation, and Forest Water Use in California Montane Watersheds

Philip Saksa,Salli Dymond,Mohammad Safeeq

doi:10.3390/f8080278

Abstract

California has recently experienced one of the worst droughts on record, negatively impacting forest ecosystems across the state. As a major source of the region’s water supply, it is important to evaluate the vegetation and water balance response of these montane forested watersheds to climate variability across the range of rain- to snow-dominated precipitation regimes. The Standardized Precipitation Index (SPI) and the Standardized Runoff Index (SRI) were used to capture the hydrologic drought signal, and MODIS vegetation indices (i.e., the normalized difference vegetation index and the enhanced vegetation index) were used to evaluate the vegetation and evapotranspiration response in three headwater catchments. The study catchments comprised a low elevation rain-dominated site (Caspar Creek) on the northern California coast, a mid-elevation site with a mix of rain and snow (Providence Creek) in the California Sierra Nevada, and a high elevation snow-dominated site (Bull Creek) in the Sierra Nevada. Lowest SPI values occurred in the third drought year of 2014 for all sites. Lowest SRI was in 2014 for Caspar, but in 2015 for Providence and Bull, reflecting differences in snowpack-delayed runoff and subsurface storage capacity between the lower and higher elevation watersheds. The most accurate water balance closure using evapotranspiration estimates from vegetation indices was within 10% of measured precipitation at snow-dominated Bull. The rain-dominated Caspar watershed had the highest vegetation index values and annual evapotranspiration, with the lowest variability over the previous 13 years (2004–2016). Vegetation index values and annual evapotranspiration decreased with increasing elevation and snow contribution to precipitation. Both snow-influenced Sierra Nevada watersheds showed elevated vegetation and evapotranspiration responses to interannual climate variability. There remains a need for institutional support to expand long-term observations in remote forested mountain watersheds to monitor and research these changing and extreme environmental conditions in source watershed regions.

Highlights

Forested watersheds in California are facing unprecedented climate extremes, which can increase the risk of invasive pests, tree mortality, and high-severity wildfire
Caspar Creek is in the Jackson Demonstration State Forest on the northern coast of California, while Providence and Bull Creeks are in the southern Sierra Nevada as part of the Kings River Experimental Watersheds (KREW)
Climate variability and extremes are expected to increase in the coming century, as California is experiencing one of the worst extended droughts in modern history

Summary

Introduction

Forested watersheds in California are facing unprecedented climate extremes, which can increase the risk of invasive pests, tree mortality, and high-severity wildfire. As per the recent estimates, there were 102 million trees that died in the Sierra Nevada during 2010–2016 [1]. These forested regions are a major source of water for ecosystem, hydropower, irrigation, and municipal uses [2]. With source watersheds playing such a critical role in the regulation and supply of water resources, it is important to evaluate the vegetation and water balance response of California forested montane watersheds to the recent climate variability across the transition from rain-dominated to snow-dominated precipitation regimes

Objectives

Methods

Findings

Discussion

Conclusion