A newly Integrated Hydrological Modelling System, IHMS has been developed to study the impact of possible climate change, change in land use and change in water management on groundwater and seawater intrusion into coastal areas. The system comprises three models combined, but can also run separately. This system is designed to represent the way that integrated water management should work i.e. the impact of change in climate, change in land use and change in abstraction on stream/river flows, drainage flow, groundwater flow, groundwater levels and seawater intrusion in coastal areas. The system comprises three packages: the DiCaSM, MODFLOW (96 and 2000) and SWI models. In addition to estimating all water balance components, the Distributed Catchment Scale Model DiCaSM, produces the recharge data that are used as input to the groundwater flow model of the US Geological Survey, MODFLOW. The latter would subsequently produce the head distribution and groundwater flows that are used as input to the Sea Water Intrusion model, SWI. Therefore, any changes in land use, rainfall, water management, abstraction, etc. at the surface will be first dealt with by DiCaSM and subsequently by MODFLOW and then by SWI. The three models operate at different spatial and temporal scales and a facility (interface utilities between models) to aggregate/disaggregate input/output data to meet a desired spatial and temporal scale was developed allowing smooth and easy communication between the three models. Since the two models MODFLOW and SWI are published and are in the public domain, this paper will focus on the newly developed unsaturated zone distributed catchment scale, DiCaSM model as well as the interfacing utilities between the three models. \ud \ud The newly developed, DiCaSM model simulates the hydrological processes of rainfall interception, evapotranspiration, surface runoff, infiltration, soil water movement in the root zone, plant water uptake, crop growth, stream flow and groundwater recharge. The model requires distributed input data sets of rainfall, land use, soil types and digital terrain information. The model accepts either distributed or non-distributed climate data. DiCaSM produces distributed and time series output of all water balance components such as potential evapotranspiration, actual evapotranspiration, rainfall interception, infiltration, plant water uptake, transpiration, soil water content, soil moisture deficit, groundwater recharge rate, stream flow and surface runoff. This paper focuses on the details of the hydrological processes and the relevant equations used in DiCaSM as well its interfaces to the MODFLOW and SWI models. In addition, the paper reports preliminary limited tests of the DiCaSM model related to the ability of the model to predict soil moisture of surface and subsurface soil layers as well as groundwater levels. The latter demonstrates how the groundwater recharge calculated from DiCaSM can be used as input into the ground water model MODFLOW using aggregation and disaggregation algorithms (built into the interface utility). SWI has also been run successfully with hypothetical examples and was able to reproduce the results of some of the original examples of Bakker and Schaars (2005). In the subsequent papers, examples of applications on different catchments will be reported. \u
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