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 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. 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 evi- dent in 20 catchments located in three main ecoregions of the continent. Runoff ratio decreased across the catchments with non-stationary hydrologic response with the exception of one catchment in northern Australia. Annual runoff ratio sensitivity to annual fractional vegetation cover was similar to or greater than sensitivity to annual precipitation in most of the catchments with non-stationary hydrologic response indicating vegetation impacts on streamflow. We use precipitation– productivity relationships as the first-order control for ecohydrologic catchment classification. A total of 12 out of 20 catchments present a positive precipitation–productivity relationship possibly enhanced by CO2 fertilization effect. In the remaining catchments, biogeochemical and edaphic factors may be impacting productivity. Results suggest vegetation dynamics should be considered in exploring causes of non-stationary hydrologic response.

Figure 6. (a) Global pattern of annual productivity (Ftot ) and mean annual precipitation relationship. While precipitation is the primary factor for vegetation growth in water-limited sites, productivity reaches an asymptote in humid areas or decreases (e.g., tropical forests) with increases in precipitation due to biogeochemical or edaphic constraints. The grey region corresponds to catchments in which productivity is insensitive to inter-annual precipitation variability. (b) A conceptual framework for characterizing changes in runoff ratio to changes in annual precipitation and vegetation productivity (Ftot ) in relation to the catchment’s hydroclimatic condition. In group (A) catchments, a positive relationship between annual precipitation and productivity exists and annual ET changes in relation to productivity depend on the dominance of structural control (increases in LAI, class A1) versus physiological control (decreases in stomatal conductance, class A2) in controlling productivity. In group (B) catchments, an inverse relationship between precipitation and productivity exists and productivity is likely constrained by biogeochemical factors. In B1 catchments, negative ET and productivity relation indicate productivity is likely con- trolled by nutrient availability as drier conditions induce nutrient mineralization. In B2 catchments, light availability and lower temperature reduce ET. In group (A) catchments, runoff ratio would increase as productivity increases, while in group (B), runoff ratio will likely decrease with increasing productivity (decreases in precipitation). Depending on the dominance of limiting resources, precipitation–productivity may shift between the two regimes. (c) The flowchart illustrates the classification procedure. The classification starts by assessing the correlations between annual precipitation and Ftot and then annual ET and Ftot in a catchment.