Hydrospatial assessment of streamflow yields and effects of climate change: Snowy Mountains, Australia
Hydrospatial analyses of catchment topographic indices for 112 unregulated (unimpaired) gauging stations show that mean catchment elevation is the primary control on annual precipitation, runoff depth, runoff coefficients and evapotranspiration in the Snowy Mountains. Catchments with mean elevations greater than 1850 m show a steep increase in yield over the trend for lower elevation catchments and have runoff coefficients greater than one. Precipitation undercatch because of high winds and winter snowfall is the cause for this unusual situation, with deep accumulations of blown and drifted snow contributing significantly to runoff from small, high elevation catchments. Climate change effects on precipitation, runoff, runoff coefficients and the timing of peak snowmelt discharges vary across an elevational gradient. Annual precipitation shows strongly significant declines of up to 11.0 mm yr−1 from 1944 to 2009, with the magnitude of precipitation declines increasing with increasing elevation. Lower elevation catchments show greater sensitivity to drought than higher elevation catchments, exhibiting sharp declines in annual runoff coefficients due to smaller average differences between evapotranspiration and precipitation, and switching from energy (demand) to supply (precipitation) limited water balances. Climate change effects on the timing of peak winter-spring (June to November) snowmelt discharges for the highest elevation gauged catchments in Australia are pronounced with average shifts toward earlier peak discharges of 6.2 and 4.0 days per decade for the Snowy and Geehi Rivers, respectively. A lapse rate model using elevation as a substitute for temperature change highlights the sensitivity of mean annual runoff coefficients in the Snowy Mountains to changes in mean annual temperature, declining by 15% and increasing by 17% per degree centigrade rise and fall, respectively. Runoff coefficient sensitivity is driven by elevation (temperature) driven controls on the proportion of precipitation falling as snow vs. rain, combined with decreasing evapotranspiration with increasing elevation. Temperature (elevation) driven decreases in evapotranspiration resulting from changes in rain-snow precipitation balances, widespread snowpack accumulation and largely treeless catchments dominated by alpine vegetation during cool phases of the last glacial cycle offer a simple but comprehensive explanation for the greater runoff volumes in the Murray-Darling basin from the SE Australian highlands preserved by palaeochannels considerably larger than present river systems.