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Hydrological Modelling of canal command using Remote Sensing and GIS
Interception and Evapotranspiration Component (et) The interception process is modelled by introducing interception storage, expressed as a function of leaf area index (Jensen, 1983). The actual evapotranspiration is calculated based on the potential evapotranspiration using the Kristensen and Jensen model (Kristensen and Jensen, 1975). The actual evapotranspiration rate, consisting of the sum of actual transpiration and soil evaporation, is further adjusted according to vegetation density and water content in the root zone. Leaf area index and root depth need to be specified as a function of time for the actual evapotranspiration calculations. The actual transpiration is calculated using the following relationship:  ---------------------(1)
Where, Eat = actual transpiration; f1(LAI) = leaf area index function; f2(q) = soil moisture function; RDF = root distribution function; and Ep = potential evapotranspiration.  ----------------------(2)
where, qf = volumetric moisture content at field capacity; qw = volumetric moisture content at wilting point; q = volumetric moisture content; and C3 = empirical parameter, mm/day. Unsaturated Zone Component (Uz) Soil moisture distribution in the unsaturated zone is calculated by solving the one-dimensional Richards' equation. Extraction of moisture for transpiration and soil evaporation is introduced via sink terms at the node points in the root zone. Infiltration rates are found by the upper boundary that may be either flux controlled or head controlled. The lowest node point included in the finite difference scheme depends on the pyretic surface level, and allowance is made for the unsaturated zone to disappear in cases where the pyretic surface rises to the ground surface. Saturated Zone Component (Sz) The ground water flow is modelled using an implicit finite difference solution of the two-dimensional non-linear Boussinesq equation for an unconfined aquifer. General Description of Study Area Pilot area of the 6 main canal command of Damodar Irrigation Project has been chosen as the study area, situated in the western part of West Bengal, India. Study area comprising total of 548 sq. km. The average annual rainfall in the command is about 1400 mm, spread over 75 rainy days. The relative humidity ranges from 65% to 95% whereas temperature varies from 11.4 to 41.7 degree Celsius. Paddy is the main crop in both Kharif and Rabi seasons, whereas vegetables are grown in few patches in summer season. On an average, three irrigations are provided in each cropping season (Kharif and Rabi). Data and Methodology Data Requirement and Collection Since the present study involves hydrological modelling of the irrigated command, the data requirements were large. The map of the 6 main canal system showing longitudinal section, details of irrigation structures, network of canals and topographic map of its command were collected from the Irrigation and Waterways Department, Government of West Bengal. Daily flow releases at system source and daily flow rates at two locations in the main canal and at three distributaries were also collected for Kharif irrigations of 1999 from the above source. The soil survey map of the command was obtained from the National Bureau of Soil Survey and Land Use Planning, Research Station, Calcutta. The RS land use map and soil map were prepared through three dates RADARSAT SAR imagery (16th July, 1st August and 25th August 1999) and IRS IC LISS-III imagery for December 1998 respectively. The climatic data such as daily rainfall and pan evaporation of three meteorological stations were obtained from the Department of Agriculture, Government of West Bengal. The leaf area index was measured by canopy analyser. Besides these, soil samples were collected from different locations (for different soil types) and physical properties of soils such as textural classification, hydraulic conductivity and soil moisture characteristic curves were determined experimentally. The monthly groundwater table depth was measured at 19 different locations in the command and processed to prepare the pre and post-monsoon groundwater table maps. |