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Previous studies have reported relationships between mean annual climatic variables and mean annual leaf area index (LAI), but the seasonal and spatial variability of this relationship for different vegetation cover types in different climate zones have rarely been explored in Australia. The authors developed simple models using remotely sensed LAI data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and gridded climatic data from the Australian Water Availability Project. They were able to relate seasonal and annual LAI of three different land cover types (tree, pasture, and crop) with climatic variables for the period 2000-09 in the Goulburn-Broken catchment, Australia. Strong relationships were obtained between annual LAI of crop, pasture, and tree with annual precipitation ( R2 = 0.70, 0.65, and 0.82, respectively). Monthly LAI of each land cover type also showed a strong relationship ( R2 = 0.92, 0.95, and 0.95) with the difference between precipitation P and reference crop evapotranspiration (PET; P − PET) for crop, pasture, and tree. Independent model calibration and validation showed good agreement with remotely sensed MODIS LAI. The results from the application of the developed model on the future impact of climate change suggest that under all climate scenarios crop, pasture, and tree showed consistent decreases in mean annual LAI. For the future climate change scenarios considered, crop showed a decline of 7%-38%, pasture showed a decline of 5%-24%, and tree showed a decline of 2%-11% from the historical mean annual. These results can be used to assess the impacts of future climatic and land cover changes on water resources by coupling them with hydrological models. [ABSTRACT FROM AUTHOR]

Overlying the challenge of managing within natural hydroclimatic variability is the likely modification of runoff variability along with average runoff due to anthropogenic enhancement of greenhouse gas concentrations. In this paper analytical models are developed in which runoff mean and variability, the latter defined by the variance (or standard deviation) of annual runoff, are related to the variances and the covariance of annual precipitation and potential evapotranspiration, and the aridity index (mean annual potential evapotranspiration divided by mean annual precipitation). The method was validated using observed runoff data for 699 worldwide catchments. It was concluded that combining the Schreiber function, which relates the ratio of annual actual evapotranspiration to annual precipitation, with the analytical models provided satisfactory estimates of observed annual runoff mean and interannual variability. It was also concluded that estimates of annual runoff variability based on the simplified model of Koster and Suarez were unsatisfactory. By way of illustrating the new methodology, the approach was applied to projected annual values of precipitation from the Hadley Centre Global Environment Model version 1 (HadGEM) and it showed that considerable changes in reservoir yield are likely to occur if climate change projections of precipitation from HadGEM are realistic. Finally, further simplifications of the equations, based on the Schreiber function, are developed to estimate the mean and standard deviation of annual runoff that allow climate analysts to estimate the impact of potential climate changes on annual runoff characteristics and reservoir yield performance without having to resort to the calibration and application of a rainfall-runoff model or rely on the runoff output from general circulation models to examine such characteristics. [ABSTRACT FROM AUTHOR]

Research into the role of catchment vegetation within the hydrologic cycle has a long history in the hydrologic literature. Relationships between vegetation type and catchment evapotranspiration and runoff were primarily assessed through paired catchment studies during the 20th century. Results from over 200 paired catchment studies from around the world have been reported in the literature. Two constraints on utilizing the results from paired catchment studies in the wider domain have been that the catchment areas studied are generally (1) small (<10 km2) and (2) from a narrow range of climate types. The majority of reported paired catchment studies are located in the USA (∼47%) and Australia (∼27%) and experience mainly temperate (Köppen C) and cold (Köppen D) climate types. In this paper we assess the impact of vegetation type on mean annual evapotranspiration through a large, spatially, and climatically diverse data set of 699 catchments from around the world. These catchments are a subset of 861 unregulated catchments considered for the analysis. Spatially averaged precipitation and temperature data, in conjunction with runoff and land cover information, are analyzed to draw broad conclusions about the vegetation impact on mean annual evapotranspiration. In this analysis any vegetation impact signal is assessed through differences in long-term catchment average actual evapotranspiration, defined as precipitation minus runoff, between catchments grouped by vegetation type. This methodology differs from paired catchment studies where vegetation impact is assessed through streamflow responses to a controlled, within catchment, land cover change. The importance of taking the climate type experienced by the catchments into account when assessing the vegetation impact on evapotranspiration is demonstrated. Tropical and temperate forested catchments are found to have statistically significant higher median evapotranspiration, by about 170 mm and 130 mm, respectively, than non-forested catchments. Unexpectedly, cold forested catchments exhibit significantly lower median evapotranspiration, by about 90 mm, than non-forested catchments. No significant difference was found between median evapotranspiration of temperate evergreen and deciduous forested catchments though sample sizes were small. Temperate evergreen needleleaf forested catchments were found to have significantly higher median evapotranspiration than evergreen broadleaf forested catchments though sample sizes were small. The significant temperate forest versus non-forest difference in median evapotranspiration was found to persist for catchments with areas <1,000 km2, but not for catchments with areas ≥1,000 km2, which is consistent with the suggestion that the vegetation impact on evapotranspiration diminishes as catchment area increases. In summary, the results presented here are consistent with those drawn from reviews of paired catchment results. However, this paper demonstrates the value of a diverse hydroclimatic data set when assessing the vegetation impact on evapotranspiration as the magnitude of impact is observed to vary across climate types. [ABSTRACT FROM AUTHOR]

Continental differences in the variability of annual runoff were reassessed using an expanded precipitation database and an improved methodology for allocating precipitation and runoff stations to Köppen climate zones. Application of the new Köppen zone allocation methodology resulted in changes in Köppen zone for 44% of runoff stations. Statistical analysis of the data, stratified by continent and Köppen climate zone, confirmed the presence of continental differences in the variability of annual runoff and the conclusions of previous research. Variability of annual runoff for temperate Australia, arid southern Africa and temperate southern Africa was found to be higher than that for other continents across the same climate zones. In this analysis a more consistent hemispheric difference in the variability of annual runoff was observed, with variability being higher in the Southern hemisphere. Previously suggested causes of the continental differences in the variability of annual runoff were also confirmed in this re-analysis. Statistically significant continental differences in the variability of annual precipitation were observed and the influence of large-scale circulation systems on precipitation variability is more apparent in this analysis than in previous work. Continental differences in the variability of annual precipitation could not account for all of the continental differences in the variability of annual runoff. The distribution of evergreen and deciduous vegetation in temperate regions remains a potential cause of the runoff variability differences. Additional potential causes identified are: continental differences in the percentage of forested catchment area, and continental differences in mean annual daily temperature range. [Copyright &y& Elsevier]

The importance of El Ni&ntildo;o-Southern Oscillation (ENSO) to annual precipitation variability is assessed by dividing stations into regions of ENSO and non-ENSO influence as defined by Ropelewski and Halpert and then comparing these regions by Köppen climate type for significant differences in the zone average of the variability of annual precipitation. Of the 13 Köppen climate types that had 10 or more precipitation stations in both ENSO and non-ENSO influenced regions, 8 had significant differences in the zone average of the variability [defined as the coefficient of variation (C[sub υ])] of annual precipitation with the variability of annual precipitation being higher in the ENSO than the non-ENSO zone in every case. The range of increase in the C[sub υ] of annual precipitation was generally 5%-25%. These results confirm earlier findings in relation to the variability of annual runoff and also are consistent with earlier findings in relation to the variability of annual precipitation. However, the impact of ENSO on variability of annual precipitation of 5%-25% reported in this note is considerably less than the 33%-50% reported previously. [ABSTRACT FROM AUTHOR]