~ 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.
For the first scenario, maximum groundwater recharge of 624mm was achieved at 10% forest cover, only marginally higher (by 0.06mm) than that with the current forest cover of 8.5 %. Interestingly, achieving the target of 33% forest cover resulted in reduced groundwater recharge by 7.94mm. The loss due to evapotranspiration (ET) for 33% forest cover was 14mm more, as compared to that with the current forest cover.
Through the second pathway, maximum groundwater recharge of 640mm was achieved at 55% forest cover. The target 33% forest cover increased groundwater recharge by 15mm when achieved through planned afforestation. Through this pathway, the ET loss for 33% forest cover was 9mm more than that with the current forest cover.
ET compensated groundwater recharge (groundwater recharge minus ET loss) for both the pathways was compared. The second pathway wins by a significant margin of 22mm at 33% forest cover. For all the four river basins, the maximum groundwater recharge value was at significantly higher forest cover for the second pathway than the first. This highlights the importance of understanding the hydrological impact of increasing forest cover.
Figure 5: Central Indian Highland’s groundwater recharge over the range of forest cover from 2% to 75% of the landscape. Graph (A) represents the groundwater recharge for the basin mean pathway for increasing forest cover while Graph (C) represents groundwater recharge optimized pathway to increasing forest cover. Graphs (B,D) subtract the increase in evapotranspiration from groundwater recharge to represent water losses from the landscape because of increasing forest cover for the basin mean pathways (B) and groundwater recharge optimized pathway (D). D33 indicates the change from current forest cover (solid vertical line) to forest cover at 33% (dashed vertical line) while Dmax indicates the change from current forest cover to the maximum (dotted vertical line).
Forests share a sophisticated relationship with the groundwater. Increasing forest cover results in higher groundwater recharge with the expense of water loss through ET. Forests can reduce basin water yield that is needed for irrigation as a result of ET and reduced peak flow. However, reduced peak flow fills reservoirs slowly and make water available for rabi season irrigation. On a regional scale and in the long run, forests can recycle evapotranspiration loss by increasing rainfall.
Paddy agriculture, on the other hand, increases depression storage and reduces the infiltration rate. Increasing forests does just the opposite by increasing infiltration and reducing depression storage. The depth of water in paddies acts as a buffer that allows continual infiltration of water even though the rate is slow, while forests primarily rely on fast infiltration during storm events. Paddy had a significant role to play in balancing the hydrology of this landscape by maintaining the depression storage. Therefore, optimal groundwater recharge is achieved when converting non-paddy agriculture land to forest, where increased infiltration more than makes up for the loss of depression storage. Alternatively, crop diversification away from paddy to alternative cereals such as millet, sorghum and maize could be effective when the focus is on cultivation methods that improve either or both depression and infiltration storage.
The first pathway indicated that lack of planning would yield no hydrological benefit and negatively affect agriculture production and groundwater recharge. However, optimised reforestation that takes into account the hydrological impacts on a landscape could improve groundwater recharge and increase net benefits. Conclusively, increasing forest cover in CIH can improve groundwater recharge when strategically planned.
Original paper: Clark, B., DeFries, R., & Krishnaswamy, J. (2021). India’s Commitments to Increase Tree and Forest Cover: Consequences for Water Supply and Agriculture Production within the Central Indian Highlands. Water, 13(7), 959.
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