Towards model adequacy for identifying the impacts of climate and land use on catchment hydrology

Land use and climate change/variability can have a major impact on catchment hydrology and these impacts can be strongly interrelated. This paper is part of a study that has an ultimate objective to isolate the impacts of land use change on streamflow from those of climate variation in the upper Murrumbidgee catchment, south-eastern Australia. The aim is to investigate the consistency or variability of catchment response and model parameters and performance over time and space. Subcatchments located in the upper part of the Queanbeyan (at Tinderry), Cotter (Gingera), Goodradigbee (Brindabella), Molonglo (Burbong) and Orroral (Crossing) Rivers with daily rainfall-discharge data for 40 years or more and minimal impact from dams have been selected for the study. As often occurs in non-experimental catchments, the land use in these catchments is closely correlated with climate differences, making a classic paired catchment study impossible. Instead, future research will focus on determining under what conditions the model performs well, and investigating whether a land use signal can be detected in the resulting subset of the data. For this purpose, both data- and model-based analyses of the dynamic relation between rainfall and runoff for these subcatchments are presented. Prior to performing the analyses, the rainfall data from selected stations have been checked and corrected to reduce the impact of errors in the areal rainfall estimates. Data analysis techniques (e. g. trend analysis, deconvolution and baseflow filtering) are used to assess the temporal and spatial variation in the hydrologic response characteristics for each site. The lumped conceptual rainfall-runoff model IHACRES CMD (Catchment Moisture Deficit) version is applied to the subcatchments to assess the adequacy of the model response in representing the impact of weather patterns on streamflow. A number of performance criteria have been used to evaluate the performance of the model in each calibration period using a multi-criteria approach. Data-based analysis shows that there has been a significantly stronger decline in streamflow compared to rainfall in all subcatchments after 1990. This decreasing trend in streamflow was more prominent at Burbong and Tinderry, with these subcatchments having lower and more variable storage capacity (the combined surface and sub-surface storage capacity of the catchment inferred from the data- base analysis) than the other subcatchments. The model-based analysis revealed that Tinderry required at least a 6 year length of record to stabilize performance statistics and yield parameter consistency in calibration. Subsequently, an 8 year calibration period was used for all subcatchments. Gingera and Brindabella showed good model performance for all performance indicators, while Tinderry, Burbong and Orroral Crossing showed poor to average model performance in R-2, R-ln(2) and bias. The reduction in performance in R2 for Tinderry and Burbong subcatchments was due to the poor fitting to the peaks for both large and small streamflow events, with the model underestimating the highest flow peaks, and overestimating smaller peaks. This suggests that the effective rainfall and/or the quick flow volume for large events are underestimated in the IHACRES model for these catchments. Further work will be needed to improve model performance for Tinderry and Burbong subcatchments in order to separate the impacts of climate variations and land use change on hydrological response. An appropriate model structure having a variable partitioning between quick and slow flow components is under consideration. Different drivers of the variability in the partitioning are being explored, including relating the slow flow volume to effective rainfall depth, seasons and rainfall depth. In addition, modifications to the CMD module of IHACRES are being investigated to improve the estimation of the effective rainfall for large streamflow events.