The Seasonal Water Balance of Western-Juniper-Dominated and Big-Sagebrush-Dominated Watersheds

Nicole Durfee,Carlos G Ochoa

doi:10.3390/hydrology8040156

Abstract

The combined impacts of woody plant encroachment and climate variability have the potential to alter the water balance in many sagebrush steppe ecosystems in the Western USA, leading to reduced water availability in these already water-scarce regions. This study compared the water-balance characteristics of two adjacent semiarid watersheds in central Oregon, USA: one dominated by big sagebrush and one dominated by western juniper. Precipitation, springflow, streamflow, shallow groundwater levels, and soil moisture were measured. The potential evapotranspiration was calculated using the Hargreaves–Samani method. Potential evapotranspiration and a water-balance approach were used to calculate seasonal actual evapotranspiration. The shallow aquifer recharge was calculated using the Water-Table-Fluctuation-Method. Evapotranspiration, followed by deep percolation, accounted for the largest portion (83% to 86% of annual precipitation) of water output for both watersheds. Springflow and streamflow rates were generally greater at the sagebrush-dominated watershed. Snow-dominated years showed greater amounts of groundwater recharge and deep percolation than years where a larger portion of precipitation fell as rain, even when total annual precipitation amounts were similar. This study’s results highlight the role of vegetation dynamics, such as juniper encroachment, and seasonal precipitation characteristics, on water availability in semiarid rangeland ecosystems.

Highlights

Many areas of the Western US are facing severe drought conditions that have dramatically decreased the levels of water available for numerous production and ecological functions
This research examined the seasonal variability of various water budget components This research examined the seasonal variability of various water budget components and shallow aquifer recharge and shallow aquifer in cool-climate rangeland ecosystems in a semiarid region in central Oregon, USA
This study examined seasonal water balance and subsurface flow dynamics in two rangeland watersheds, one dominated by western juniper and one dominated by big sagebrush, in central Oregon, USA

Summary

Introduction

Many areas of the Western US are facing severe drought conditions that have dramatically decreased the levels of water available for numerous production and ecological functions. Long-term deficits in available water associated with prolonged droughts and a changing climate threaten production systems and ecosystem resilience in many arid and semiarid regions worldwide. Seasonal timing of precipitation is a key factor influencing the water balance, and water availability, in sagebrush steppe ecosystems [6], in regions with already variable precipitation patterns [7]. Shifts in plant functional types have been shown to alter the water balance of dryland ecosystems [8]. In the sagebrush steppe ecosystems of the Great Basin, the combined effects of woody plant encroachment [10,12] and increased temperatures and precipitation variability associated with climate change [2,13] can alter the seasonal water balance

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Discussion

Conclusion