Towards better river basin planning - Improving the links between hydrologic, economic and agricultural models

The New South Wales (NSW) Department of Land and Water Conservation (DLWC) has developed a hydrologic river basin planning model known as the Integrated Quantity and Quality Model (IQQM). This model operates on a continuous daily time step over periods ranging up to one hundred years. IQQM has been implemented in many river valleys in both NSW and Queensland. IQQM is primarily used to investigate the impacts of water management policies on competing water users. This includes the environment. The model is also used to audit the usage of water within NSW?s western flowing valleys to verify that current usage is complying with constraints imposed to reduce the degradation of rivers within the Murray?Darling Basin. These constraints, known as the ?Murray?Darling Basin Water Cap?, impose a limit on the amount of water that can be extracted from rivers within the basin. IQQM currently contains two simple models for soil moisture accounting for simulating crop water requirements and subsequent water usage. The model considers antecedent conditions, current available water resources and expected resources, to determine an area to plant. This decision of what area to plant is made on specified days, either once or twice in each growing season within a water?year (a water?year generally runs from July to June) and applies to the whole mix of crops that is grown in each growing season. The model does not take into account the different planting dates of various crops and any increases or decreases in planted areas as the availability of resources changes. The impacts of water availability and climate on the growth of crops are not taken into consideration. In order to investigate the tradeoffs between water users and the environment, outputs from the current IQQM must be post-processed before being used in economic analyses. This often involves several iterations between the models before the economic analysis is valid. To streamline this process and to improve the linkage between IQQM and economic models, it was decided to develop a crop yield module in IQQM that considers varying ?windows of opportunity? for planting, considers crop growth based on degree?days and determines how water shortages affect crop yields. The module also simulates the ways farmers change crop areas and mix in response to past and present resource availability.