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Basin-wide Holistic Integrated Water Assessment (BHIWA) Model
Basin-wide Holistic Integrated Water Management (BHIWA) Model is especially developed to provide an integrated computational framework for a basin level assessment of water resources with a view to evaluate water sector polices. The model considers the entire land phase of the hydrologic cycle and is capable of depicting human impacts such as changes in land and water use, as also impacts of water storage and depletion through withdrawals for various water uses and inter basin water transfers. The model takes into consideration complex interaction between numerous factors including surface and groundwater, land use and natural water supply, storage and water withdrawals and returns, through separate water balances for surface and groundwater as well as an overall water balance.
The model is especially useful for understanding existing as well as future water availability; assessing future water needs under different scenarios, and for analyzing impact of different policy options for an integrated and sustainable development of resources.
The model can be calibrated making use of data for the past or present conditions for the given basin. Once the model is calibrated, the user can proceed to simulate and analyze alternate scenarios of future development and management of resources. Scenarios can be developed in the model in terms of changes in land use, crop areas under rainfed and/or irrigated agriculture, cropping patterns, irrigation efficiencies, imports and exports of water, surface (reservoirs) storage, proportion of surface and groundwater withdrawals, etc. By simulating past conditions of limited water use in the basin, the model can also help in setting up minimum reference flows for maintenance and enhancement of river ecology and environment. Comparison of such flows with projected future river flows can help in setting limits on surface and ground water withdrawals, including extent of lowering of groundwater tables to meet prescribed "environment flow" requirements.
Composition of BHIWA Model
The Basin-wide Holistic Integrated Water Management (BHIWA) model as evolved for CPSP has 9 computation modules. The model is developed in Microsoft EXCEL software and has a number of spreadsheets. The model works, initially, in the calibration mode using the observed data. After obtaining a generally satisfactory calibration mode, it is worked as a tool for assessing the possible status of the basin, under difficult scenario in the simulation mode. This process is depicted in Figure. For using the model, a river basin is first to be divided into hydrologically homogeneous sub- basins and each sub- basin into a number of land parcels each depicting a particular category/sub-category of land use. The model accommodates a maximum of 5 sub- basins and each sub- basin can be divided into a maximum of 25 land parcel types. The hydrologic computations are first performed for each land parcel in terms of water depth in millimeter over the area and then aggregated in volume units (million cubic meters) at the sub- basin level.
[BHIWA Model Software (Windows XP version)]
Natural (Hydrologic) Module 1 : Computation of actual ET, quick runoff and natural recharge.
The model calculates water balances for the upper and lower zones viz. soil profile and groundwater system for each land parcel, given soil moisture holding capacity of the parcel, and area averages of rainfall, and reference evapo-transpiration for the sub-basin. The soil profile component of the model partitions the rainfall into actual evapo-transpiration (ET) and excess water. The actual ET is calculated as a function of potential ET and the actual moisture availability, as proportion of the root-zone soil moisture capacity for each land use type. These functional relations depict how the actual ET reduces with reduction of soil moisture availability, or indirectly the tension in the root zone. The excess water is further divided into deep percolation (natural recharge to groundwater) and quick runoff from land areas to the river. The quick runoff from all land parcels is aggregated into a single entity to represent natural contribution from rainfall to the river system. Likewise, natural recharge to groundwater under various land categories is lumped into a single groundwater entity representing the natural contribution of rainfall to the groundwater.
Module 2 : Computation of irrigation withdrawal.
This module calculates the requirement of additional water for each of the irrigated land parcels using data from previous module on shortfalls to meet the Potential Evapo-transpiration (PET) requirements. Net and gross irrigation requirements are computed source- wise using data on irrigation system efficiencies and proportion of surface water irrigation. For parcels having paddy crop, net water requirements are calculated taking into account user prescribed monthly percolation. Estimates of withdrawals for irrigation are arrived at finally considering "deficit irrigation" specified if any.
Module 3 : Computation of irrigation returns.
These are computed separately for surface water and groundwater irrigation systems using user specified information on potential return from the total water withdrawn, in excess of the AET and that part of the wasteful return, that will be lost as ET from swamps/waterlogged areas with in cropped lands. The difference between the potential and the wasteful return is further divided into the components returning to surface and groundwater system.
Module 4 : Accounting for Evapo-transpiration (ET) by sector.
This module is designed for accounting ET by different use sectors. This is achieved through sectoral identification of each land parcel type. Agriculture land parcels are further divided into rain-fed and irrigated parcels. Parcel ET is designated as beneficial, if it is productive from consideration of sectoral water use. Otherwise it is classified as non-beneficial.
Module 5 : Computation of domestic and industrial withdrawals, use and returns.
In calibration mode, this module is run on directly fed data. However, in simulation mode, domestic and industrial (D&I) module is used first to project population and water requirements in the targeted "future" year from the user given information on base year, intermediate blocks, population growth rates and proportion of urban population to total population. Withdrawals are next computed in the model using rural and urban water supply norms and source-wise proportion of supplies. Information on consumptive use fraction and returns is used to calculate the total return as well as its components to surface and groundwater systems.
Module 6 : Computation of the river water balance.
It aggregates all inputs to the river including quick run off, base flow and returns from irrigation, D&I withdrawals and computes balance flow taking into account given values of storage changes and requirements of environmental flow. Provision exists to account for adjustments in surface water withdrawals through assumption of induced recharge from the river flow to groundwater in cases where the estimated groundwater withdrawal is found to be unsustainable. This module also has a provision to ensure that the river flow in any month is not less than the specified environmental flow requirement (EFR), or zero, if no EFR is specified. This is achieved through extra pumping from groundwater reservoir to take part of the demands on surface water.
Module 7 to 9 : Computation of groundwater balance.
The input part of the module facilities aggregation of input from deep percolation from natural rainfall, return from irrigation and D&I withdrawals and as well as induce recharge if any required from the river. The output components of groundwater system include base flow to river and withdrawals through pumping from ground water reservoir as also pumping into canals to meet the surface water shortages, if there be any. In the simulation mode, the module is designed to achieve a stable groundwater regime under average conditions by adjusting the initial groundwater reservoir storage. Where the total annual input to groundwater is detected to be less than the estimated withdrawals including natural out flow (base flow) to the river, there exists a provision to manually balance groundwater through artificial recharge from surplus river flows for achieving a sustainable or balanced groundwater regime. Consequences of modifications in groundwater reservoir system are carried forward to modify the river water balance.
In addition to the above modules, there are worksheets to facilitate data inputs, and generation of aggregated results in the form of tables and charts.
The model runs on a monthly time step simulating average hydrological year. In the calibration mode, however, a model can be applied either to a single year (good, average or dry) or to a sequence of years (maximum length 5 years).
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