India has committed to reducing greenhouse gas emission intensity under its nationally determined contributions (NDC), made at the United Nations Climate Change Conference in 2015 (COP21). To achieve this goal, India plans to create carbon sinks of 2.5 to 3 billion tons of carbon dioxide equivalents by increasing its forest and tree cover to 33% of its land area. The effort to increase tree cover up to 33% sits within the National Mission for a Green India (GIM), one of eight Missions under the National Action Plan on Climate Change (NAPCC) as well as earlier national forest policy goals. The GIM plans to increase tree cover on five million hectares of designated forest lands and forest on non-forest designated lands and improve tree cover on an additional five million hectares [1
]. This effort, if achieved, would ultimately result in three to five million hectares of degraded or marginal agricultural land being converted to forest or agroforestry [2
]. To minimize negative impacts on biodiversity and local pastoral livelihoods, conversion of natural or managed grasslands to forest will also need to be avoided [5
]. One of the stated goals of GIM is to improve the hydrological services within the affected landscapes. Using this as a point of departure, this paper examines the effects of converting cropland to forest cover within the Central India Highlands (CIH) to achieve 33% forest and tree cover within each river basin. It focuses on the impacts on groundwater recharge, essential for sustainable Rabi season (winter, non-monsoon season) irrigation. The CIH is selected as it contains significant forests and has rapidly increased its agriculture production and groundwater abstraction with groundwater accounting for 41% of irrigation water demand over the last decade [8
]. In addition, the CIH is a hotspot for extreme precipitation events and climate change [9
India ranks number ten in the world for forested area but only 120th in terms of the percentage of land area under forest [11
]. The Forest Survey of India (FSI) conducted in 2019 estimates a total of 807,276 square kilometers of forest and tree cover, which makes up 24.56 percent of the land area [12
]. The 2019 forest area represents an increase of 78,852 (2.4%) square kilometers over the past two decades, with the 1997 Forest Survey of India (FSI) reporting 633,397 square kilometers (19.27%) [13
]. Given these estimates, India must at a minimum, increase its tree cover by 12% over the next decade, meaning adding 32,874 square kilometers per year on average. The amount of tree cover required is approximately three times the land area proposed within the GIM’s stated goals. The magnitude of land cover change required to meet the COP21 commitments, if achieved, has the potential to significantly impact the hydrological cycle of the affected landscapes, with implications for both agricultural production and irrigation potential.
The infiltration-evapotranspiration trade-off hypothesis provides a framework for understanding the possible alteration of the hydrological cycle from reforestation and afforestation [14
]. As compared to other land cover, forests have higher rates of evapotranspiration (ET) but also have higher infiltration and groundwater recharge [15
]. The balance between these two depends on a variety of soil, geologic, and land use history, and vegetation attributes [19
]. Through greater infiltration, groundwater recharge, and evapotranspiration, forests also reduce peak flows [17
]. Likewise, forest compared to other land cover tends to have the lowest annual water yields [22
]. Much of India’ cultivable land is devoted to rice production in paddies where infiltration rates are slow, and ET is reduced in comparison to forest. Within the CIH 34% of the land area is devoted to paddy agriculture ranging from 18% to 73% per basin [24
]. Rice is grown during the monsoon season with excess water routed through surface drainage instead of percolating to groundwater. Paddy, as a widespread cultivation practice that occupies a considerable land area within each basin, has a significant impact on the amount of groundwater recharge that can subsequently be used for groundwater-based Rabi season irrigation to support multi-cropping and bust agricultural production [25
]. Consequently, forest and paddy land covers present very different dynamics in the context of the infiltration-evapotranspiration trade-off hypothesis. Conversion between these two land covers should have a substantial impact on the inter-annual dynamics of the hydrological cycle and availability of groundwater as a consequence of addressing India’s COP21 commitments.
Evidence from afforestation and reforestation studies from around the world show divergent impacts on river basins. Most report a decline in basin discharge, with differences in studies between the impact on fast runoff and baseflow [27
]. Krishnaswamy et al. 2018 [14
] report neutral to positive effects of forest cover within a basin on dry-season flow, suggesting forests play a role in the temporal dynamics of streamflow. The reduction in discharge as result of planting trees is largely attributed to the increase in ET [31
]. When agricultural land is converted, past cropping intensity and irrigation can have an impact on the ET changes resulting from planting trees. One exception to declining discharge was reported by Lacombe et al., 2016 where teak plantations replaced paddy agriculture in Laos. Reforestation can also alter dominant flow pathways and dampen streamflow response to precipitation events [32
]. Afforestation of agricultural land within the tropics has been shown to have a dramatic impact on infiltration, with between two and four-fold increases [33
]. Zhang et al. (2019) [34
] report an increase in soil hydraulic conductivity in 23-year-old reforested pine, suggesting that soil properties take time to develop post tree planting. Soil moisture has also been shown to decline after afforestation; this, however, is strongly dependent on the species, density, and phenology of the trees planted [35
]. Groundwater recharge is enhanced by the condition of the forest, with plantations providing less recharge than natural forests [36
] and conversely an increase in overland flow associated with degradation resulting from overuse [37
]. Adjustments to basin hydrology also occur over an extended period after afforestation, with Brown et al., 2013 [27
], reporting basins achieving equilibrium after 8 to 25 years and Webb and Kathuria, 2012 [28
] reporting maximum streamflow reductions after 14 years. The trade-off between increased infiltration and ET resulting from reforestation can take decades to develop and may never achieve the advantageous balance of natural forest [38
]. Afforestation and reforestation have complex impacts on river basin hydrology that play out over both temporal and spatial scales, making them difficult to predict [39
India’s total cropland area has been largely unchanged since the 1970s, at approximately 60% of the total land area [41
]. To meet the ever-growing food demand of the expanding population, India has intensified its agriculture through additional growing seasons that require irrigation. Initial investments for developing irrigated croplands were predominantly in surface-irrigation schemes. In recent years, with bore wells becoming cheaper to drill, expansion of the electrical grid, and provision of pumping subsidies, many farmers have installed bore wells [42
]. In some regions of India, this has resulted in an over-exploitation of groundwater resources and a declining water table [45
]. While India has ample water resources overall [46
], intra-annual variability can create temporal water stress that limits Rabi season irrigation [8
Knowing the balance between water loss and water gain both spatially and temporally throughout the year is crucial in determining synergies or trade-offs between agricultural production and increases in forest cover for carbon sequestration. This paper seeks to answer the following:
What would be the impact of increasing forest and tree cover within the CIH to 33% of the basin area?
What type of forest and tree cover yields the maximum groundwater recharge?
Which hydrological parameters dynamics need to be considered when planning reforestation?
To answer these questions, this paper first examines the impact of land cover on field saturated hydrological conductivity (Kfs) in the CIH. These findings are then incorporated into a modified spatial processes in hydrology (SPHY) model for five river basins whose headwaters are within the CIH. The model is then used to simulate forest cover from 2% through 75% to identify the forest cover required to maximize groundwater recharge. The paper discusses how infiltration and depression storage interact to control groundwater recharge when reforesting paddy-based agriculture landscapes. Lastly, the paper addresses implications for agriculture production and Rabi season irrigation from groundwater sources.
India’s NDC at COP21 of reducing its greenhouse gas emissions, partly by increasing tree cover, will require balancing the loss of agricultural land with increased production through intensification and irrigation of Rabi season crops to maintain the nation’s food production. Previous studies have shown that increased use of groundwater for irrigation in Northern India is not sustainable due to rapidly falling water tables [45
]. The CIH over the last decade has seen a substantial increase in groundwater abstraction for irrigation which is estimated to account for approximately 41% of irrigation water withdrawals [8
] and requires 1.2 ha of land to recharge the irrigation water demand for one hectare of multi-cropping agricultural land. The infiltration-evapotranspiration trade-off hypothesis would suggest that increasing forest cover should help with groundwater recharge at the expense of increased ET. Forests are also linked to reduced basin water yield [23
], which is currently essential to maintaining surface water irrigation schemes within the region. Much of the reduced water yield can be accounted for by the increase in ET with the remainder resulting from reductions in peak flows generated from surface runoff, which can be important in reducing the frequency and intensity of destructive floods [27
]. Forest cover can also increase baseflow, resulting in healthier river systems and delayed discharge [87
]. Forest and tree cover can also feed-back into and interact with the atmosphere to reduce temperatures and recycle ET into increased rainfall at the landscape or regional scale, as highlighted by Noordwijk 2018 [89
] in his forest-water paradigms and also demonstrated for the Western Ghats where forest ET contribution to rainfall in dry parts of Southern India is indicated [90
]. Reduced peak flows and increased baseflows would also fill reservoirs more slowly and make water available for Rabi season irrigation. Consequently, increasing forest cover within the CIH has complex hydrological interactions related to sustaining groundwater-irrigated agricultural production within the region.
The hydrological modeling carried out in this study, supported with the field data collection on Kfs
, sheds light on how increased forest cover might impact the hydrological cycle and the consequences for irrigated food production within the CIH. The field data on Kfs
shows a three-fold difference between forest Kfs
and cropping Kfs
, irrespective of soil type, with no difference between teak plantations and natural forests. Forest Kfs
roughly matches the median rainfall intensity, while agriculture Kfs
is likely to generate runoff during two thirds of the rainfall events. Paddy rice is the dominant form of agriculture and covers a large percentage (20% to 90% of the agricultural land area) and consequently important for recharging groundwater. Paddy differs from forest and other agriculture land cover due to its large depression storage. The results of the hydrological modeling show that the current landscape, dominated by paddy agriculture, has large volumes of depression storage but low infiltration rates. In comparison, increasing forest cover would greatly reduce depression storage while improving infiltration rates. The hydrological modeling shows that increased depression storage can largely cancel out the increase in infiltration rates when converting paddy agriculture to forest [91
]. Depression storage allows water to infiltrate on a continual basis, whereas with the lack of depression storage, the process of infiltration predominantly occurs during rainfall events. The depth of water in paddies acts as a buffer that over time allows significant amounts of water to infiltrate even though the rate is slow, while forests primarily rely on fast infiltration during storm events. Converting non-paddy agricultural land, which has low infiltration and depression storage, to forest results in optimal groundwater recharge.
The two pathways to reforestation or afforestation simulated in the hydrological modeling within the basins of the CIH demonstrate the need to think carefully as to where to plant trees to increase groundwater recharge. The basin mean approach demonstrates that a lack of strategic planning would yield no hydrological benefits and a decrease in groundwater recharge while also losing agriculture production. On the other hand, the groundwater recharge optimized pathway demonstrates that planting trees in the non-forested land cover other than paddy would yield considerably more groundwater recharge with intermediate forest cover showing gains in potential to increase net benefits. Both pathways result in similar losses of water to ET. The two pathways also differ in the amount of surface runoff, with the basin mean pathway increase surface runoff by ~36 mm while the groundwater optimized pathway reduces it by ~25 mm. Reducing surface runoff would have a positive effect on reducing flooding and siltation of surface water bodies. The trade-off for improved groundwater recharge of optimized increased forest cover would be the reduction in overall discharge and increase in ET. Rabi season irrigation from groundwater sources would benefit from the increase in groundwater recharge, while the increase in ET and diminished discharge would have little impact.
Planning forest cover increases in paddy agriculture areas requires balancing losses of depression storage with an increase in infiltration rates to achieve beneficial hydrological dynamics [92
]. The impact of the method used to reforest paddy on the infiltration rate and depression storage will also influence the time it takes for the site to reach a new hydrological equilibrium. Methods that focus on restoring hydrological function by increasing infiltration should reduce the time to the new equilibrium. A loss in depression storage as a consequence of converting paddy to forest will result in a reduction of groundwater recharge until the forest can improve the infiltration and restore the groundwater recharge. To increase tree cover on paddy while minimizing negative impacts on groundwater recharge, infiltration rates in paddy would need to increase to compensate for the loss of depression storage.
This study focuses on the impact of forest cover on infiltration and its linkages to groundwater recharge at the landscape scale. The analysis provides insight into the trade-off between infiltration rate and depression storage in paddy agricultural landscape, but it has not addressed where on the landscape reforestation or afforestation could optimize groundwater recharge. Likewise, this study has not looked at the influence of forest cover on other hydrological parameters. Spracklen et al. 2012 [93
] included India as one regions of the world where forest cover has the potential to increase rainfall. Additionally, the model does not address the dynamics of the period after reforestation when soil properties are being altered by tree and understory growth with increasing infiltration, balanced with the increased ET. This period may represent a very different balance from the result presented here, as ET is likely to develop faster than the change in infiltration, especially in converted paddy. There are potentially multiple synergistic benefits from increasing forest cover in the CIH that would promote better ecological function and sustainable agriculture at the landscape scale.
While India made ambitious commitments at COP21 in setting a 33% tree cover target, current land use poses a significant challenge to achieving the aims GIM. The results of this study indicate that the hydrological aims of the GIM would be promoted by increasing forest cover, but only if balanced with losses of depression storage by preferentially retaining agricultural land with maximum depression storage such as paddy. The cost to cropland would be high, but the improved hydraulic dynamics at the landscape scale from well-planned reforestation or afforestation would help improve the sustainability of Rabi irrigation. Alternatively, there are advantages to crop diversification away from paddy to alternative cereals such as millet, sorghum and maize from both sustainability and nutritional perspectives [94
]. Here the focus should be on soil management and cultivation methods that increase either or both depression storage and infiltration rates on non-paddy cropland and non-forested lands. Agroforestry systems would also be explored as a method for increasing tree cover while promoting agricultural production [95
]. Such approaches would increase nutritional output while reducing irrigation water demand and would also have a large impact on improving groundwater recharge at the landscape level. There is potential for increased tree and forest cover to boost both food production and water availability. More research is needed to better understand the dynamics of reforestation and afforestation within paddy agriculture landscapes like the CIH.