Assessing the impact of climate variability on catchment water balance and vegetation cover

Understanding the interactions among climate, vegetation cover and the water cycle lies at the heart of the study of watershed ecohydrology. Recently, considerable attention is being paid to the effect of climate variability on catchment water balance and also associated vegetation cover. In this paper, we investigate the general pattern of long-term water balance and vegetation cover (as reflected by fPAR) among 193 study catchments in Australia through statistical analysis. We then employ the elasticity analysis approach for quantifying the effects of climate variability on hydrologic partitioning (including total, surface and subsurface runoff) and on vegetation cover (including total, woody and non-woody vegetation cover). Based on the results of statistical analysis, we conclude that annual runoff (R), evapotranspiration (E) and runoff coefficient (R/P) increase with vegetation cover for catchments in which woody vegetation is dominant and annual precipitation is relatively high. Control of water available on annual evapotranspiration in non-woody dominated catchments is relatively stronger compared to woody dominated ones. The ratio of subsurface runoff to total runoff (R-g/R) also increases with woody vegetation cover. Through the elasticity analysis of catchment runoff, it is shown that precipitation (P) in current year is the most important factor affecting the change in annual total runoff (R), surface runoff (R-s) and subsurface runoff (R-g). The significance of other controlling factors is in the order of annual precipitation in previous years (P-1 and P-2), which represents the net effect of soil moisture and annual mean temperature (T) in current year. Change of P by +1% causes a +3.35% change of R, a +3.47% change of R-s and a +2.89% change of R-g, on average. Results of elasticity analysis on the maximum monthly vegetation cover indicate that incoming shortwave radiation during the growing season (R-sd,R- grow) is the most important factor affecting the change in vegetation cover. Change of R-sd,R- grow by +1% produces a -1.08% change of total vegetation cover (F-t) on average. The significance of other causative factors is in the order of precipitation during growing season, mean temperature during growing season and precipitation during non-growing season. Growing season precipitation is more significant than non-growing season precipitation to non-woody vegetation cover, but both have equivalent effects to woody vegetation cover.