Simple model

though seasonal water availability is imperative for any country, not enough information is available to appropriately budget river water in India. This is due to the dearth of data on the factors associated with hydrology, and absence of a feasible framework to put these variables in perspective. Researchers of Goa-based National Institute of Oceanography (nio) have tried to fill up this lacuna. They have assembled a 'framework' to gauge the seasonal flow of any river (Current Science, Vol 86, No 4, February 25, 2004).

At the core of their system is the hydrological routing algorithm (hydra), a software developed at us-based University of Wisconsin. hydra can simulate river flow, both surface and sub-surface, from its source to destination. In addition, the researchers have used commonly available digital elevation models (dem) of topography, and data on precipitation and evaporation.

To weigh the viability of their framework, D Shankar along with Vidya Kotamraju and S R Shetye assessed the hydrology of the Mandovi river, which originates in the Western Ghats and flows into the eastern Arabian Sea. The researchers chose Mandovi because it flows near nio, and hence there is no dearth of information on its hydrology.

The most important task for the researchers was to calculate the discharge during July-August (peak monsoon months) and December-May (the lean season). It is important to correctly simulate both the discharges, because the former is a measure of the flood potential and the latter is an indicator of the amount of groundwater that can be tapped throughout the year; in many places, the levels of groundwater table increase with the river water seeping inside the aquifers.

A combined simulation of the results of hydra and dem and data on precipitation and evaporation tallied with the previous measures of Mandovi's flow rate -- a proof of the framework's accuracy. The simulations also indicated that seasonal changes in soil moisture played the most important role in deciding the amount of the flow. At the onset of monsoon in June, the soil in the Mandovi's catchment area is dry; hence much more of the rainwater penetrates into the soil column. This implies that a large fraction of the runoff goes into sub-surface flow, decreasing the active river flow during May and June. But the flow increases considerably during July and August, when the soil is almost saturated.

According to the researchers, the model can be used to simulate the flow of any river in the country.

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