The Context

Written by Balachander T
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The Nilgiris have seen a drastic change in land use pattern over the last century or so with shola forests and grasslands being replaced by monoculture plantations, habitations, farmlands etc. There is also an ever increasing population settled locally as well as a significant floating population that visits the Nilgiris for tourism. The pressure of all these factors affects the flow of water from the Nilgiris negatively. There has been some research in the past that has looked into these effects.

1. A study by Meher-Homji (1984) in the Nilgiris show a diminishing trend for the number of rainy days, but the conclusion is not highly affirmative since it compares a window of 4 years: 1886-1890 with another window of 4 years: 1978-1982.

2. A study by the CSWCRTI in the Nilgiris used a paired watershed study approach to look at the impact of replacing grasslands by Bluegum plantations (Sikka and Selvi, 2006). The study shows that this replacement resulted in reduction in water yield, decreased low flows and increased soil moisture losses. The reduction in water yield was 16% and 25% respectively in the first and second rotations. Such results have important implications when considered over a large scale, if such policies such as conversion of grasslands into new forest type are made. Therefore, conversion into any forest type might not yield desired results; at the same time the age of the forest and other components of the forest hydrology such as recharge into groundwater might need to be considered, which are ignored in this particular study.

Recently, interesting work in the Western Ghats has been done under the IHP programme with collaboration with NIH and ATREE. One such work describes the unique nature of hydrogeology of the Ghats in Karnataka in which much of the runoff is in terms of subsurface storm flow through large pipe network just below the surface (Putty, 2006). To classify this as base flow wouldn’t be correct since the time span is very rapid. Similar observations have been made in CSWCRTI monitored watershed in Uttaranchal, eg the Salri watershed, wherein almost 50%-70% of precipitation is reported to be infiltrating. It would be erroneous to assume this infiltrated water as entering the groundwater table since much of it flow downstream immediately as subsurface storm flow component of streams. There is also attempt at modeling groundwater flows over a sub-basin level and simulating the impact on forest land due to pumping (Sekhar et al, 2007). It has been suggested through modeling that excessive pumping from agricultural land could lead to decline in groundwater levels in the Bandipur forest area.

Given the scale of changes that have happened over the decades in the Nilgiris, prima facie it seems that massive changes in hydrological services would have occurred. It is therefore imperative that policy making is informed of the implications of such land use changes in a simple and accessible manner that is based on sound scientific method[1].

[1]          . Source: Krishnan, Sunderrajan. Forests and Hydrologic Rejuvenators: Compiling current debates to guide future action. 2008.

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