On the non-stationarity of hydrological response in anthropogenically unaffected catchments: An Australian perspective.

Increases in greenhouse gas concentrations are expected to impact the terrestrial hydrologic cycle through changes in radiative forcings (affecting precipitation and temperature) and plant physiological and structural responses. Here we investigate the nature and frequency of non-stationary hydrological response as evidenced through water balance studies over 166 anthropogenically unaffected catchments in Australia using data from the Australian network of Hydrologic Reference Stations (http://www.bom.gov.au/water/hrs/). Non-stationarity of hydrologic response is investigated through analysis of long term trend in annual runoff ratio (1984-2005). Results indicate that a significant trend (p < 0.01) in runoff ratio is evident in 19 catchments located in three main ecoregions of the continent. Runoff ratio decreased across the non-stationary catchments with the exception of one catchment in northern Australia. Non-stationarity of runoff ratio is caused by complex interactions between changes in precipitation, climate variability, plant physiological and structural responses to elevated CO 2, landscape characteristics (soil and topography), and the hydroclimatic condition of a catchment. We use precipitation-productivity relationships as the first order control for catchment classification under climate change. Monthly fractional vegetation cover from the Advanced Very High Resolution Radiometer (AVHRR) sensor is used as a measure of vegetation productivity. This first order grouping of catchments will help to generalize catchment behaviour in terms of changes in runoff ratio and vegetation productivity due to changes in precipitation. Eleven out of 19 catchments present a positive precipitation-productivity relationship possibly enhanced by CO2 fertilization effect. In the remaining catchments, nutrient limitations may be impacting productivity. The proposed classification framework provides a general guideline for projecting the likely changes in catchment water balance in response to climate variability. However, uncertainty about the terrestrial ecosystem response still remains as productivity is impacted by factors such as availability of water, nutrients and light, vegetation developmental stage, space constraint and prevalence of pests. In addition, it is expected that frequency and duration of extreme events such as fire, drought and floods to increase in future. These changes are expected to further alter ecosystem response and water availability.