~ By Satvik Parashar
A recent study lead by Dr. Benjamin Clark explores the association of groundwater recharge and infiltration for different proportions of forest cover and agricultural land in the Central India Highlands (CIH). The evapotranspiration (ET) trade-off hypothesis helps us understand how forests and croplands differ in the ways they collect and release water. Forests have higher infiltration and ground-water recharge, but also have a higher rate of evapotranspiration. On the other hand, in paddy croplands, infiltration and recharge is slow, but they have greater depression storage and reduced ET loss when compared to forests.
The study was conducted in the Central Indian Highlands spanning the states of Madhya Pradesh, Chhattisgarh, Maharashtra, Uttar Pradesh, and Rajasthan. The landscape contains nearly 8% forest and around 88% agricultural land. It is drained by five major rivers, namely Ganga, Narmada, Tapi, Godavari, and Mahanadi
Figure 1: Central Indian highlands with the five major basins delineated. Forest cover is shown in green while agriculture is in yellow derived from the European Space Agency (ESA) Land Cover 2010 data reclassified. The inset map shows the sampling area for infiltration tests and the final sampled locations. The color of the sample locations represents the land cover.
The study used hydrological modelling to determine groundwater recharge and evapotranspiration loss for each land use type and for different proportions of forest cover. Forest cover percentage ranged from 5% to 75% with intervals at 5%, along with additional two values of 2% (approximate current minimum for some basins) and 33% (India’s target COP21 NDC). Saturated hydraulic conductivity (Kfs) was used to understand the groundwater scenario for each land-use class. According to United States Department of Agriculture (USDA), Kfs is a measure of the ease with which pores of a saturated soil permit water movement. Teak plantations had the highest Kfs value of 23.2 mm/h and cropland had the least value of 6.7mm/h. Forest had a value of 20.2mm/h. Suggesting that forests and plantations allow have a higher rate at which water can move deeper into soils to replenish ground water. However, this does not account for water lost from plant use in these environments by ET, which the models need to subtract to provide management inferences. Two pathways were used to determine the hydrological impact of forest cover in CIH:
1. The first pathway analyzed hydrological change when basin mean forest cover was increased in an arbitrary, unplanned manner.
2. The second pathway involved analyzing landscape hydrology, when forest cover was increased by converting non-paddy agriculture land, so as to optimize groundwater recharge.
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