CISED Research Team: Sharachchandra Lélé, Ajit Menon, Shrinivas Badiger, Iswargouda Patil, A.K. Kiran Kumar, Rajeevakumar, Sowjanya Peddi and Lakshmikant
Collaborating Institutions: National Institute of Hydrology, ATREE, UNESCO and with the cooperation of the Karnataka Forest Department
The hydrological service benefits of tropical forests are the most poorly understood and contentious of all forest ecosystem benefits. This research study is a major, collaborative, and multidisciplinary project launched in 2002. It has attempted to understand the impacts of changes in forest cover on the hydrology, and of these hydrological changes on local communities in the Western Ghats region of Karnataka. With particular focus on differences between relatively intact forests and heavily used forests and forest plantations, the research has sought to trace the impacts of such changes on the hydrology and thereby on agricultural production, incomes and employment, and forest product benefits across different groups within settlements immediately downstream of the watersheds.
While five sites were initially selected for study, data collection and analysis could be completed only in four of them. These sites were spread across a range of rainfall regimes, ranging from ~3500mm in coastal Areangadi (Honnavar taluka) and ~2900mm in upghat Kodgibail (Siddapur taluka) to ~1000mm in Bandipur (Gundlupet taluka) and ~800mm in Arepalya (Kollegal taluka), and a corresponding range of forest vegetation types and livelihood systems.
Results from the high-rainfall Kodgibail site clearly demonstrate that conversion of natural forests to acacia plantations and heavily used tree savannahs reduces the infiltration capacity of the soils at the surface, leading to increased surface runoff. While there are differences in the evapotranspiration losses between tree savannahs and acacia plantations, in both cases there is likely to be a net reduction in the catchment’s contribution to groundwater recharge and hence to the post-monsoon flows in second order streams. Comparison of productivity and profitability of arecanut cultivation (a highly profitable crop) in valleys with different duration of post-monsoon flows showed that the valleys with longer post-monsoon flows have, on an average, greater arecanut productivity. On the other hand, farmers with greater access to forest products, particularly leaf manure and mulch, also have higher arecanut productivity. This suggests that arecanut farmers have to strike a balance between the indirect benefits from post-monsoon flows and direct benefits from the harvesting of forest products. The variation in forest tenure regimes across the landscape makes it impossible for farmers to make individual tradeoff decisions, and the absence of a village-level mechanism for community management currently makes it difficult to understand and resolve such tradeoffs at the community level.
Results from the drier site in Bandipur show how the technological context can influence the direction of the impact. The analysis of catchment hydrology showed reductions in surface soil infiltration capacities under heavy use as in the case of Kodgibail, and this factor dominates over any reductions in evapotranspiration due to sparser vegetation, resulting in quicker and higher runoff in the heavily used catchments. But interestingly, this change seems to actually benefit the community immediately downstream of the tank, as it increases the probability of timely filling of the irrigation tank built across the stream. And the effects are non-linear: A decrease in the runoff coefficient from 0.18 (which is the higher end of the estimated catchment response due to heavy forest use) to 0.12 will result in a change in December tank-filling probability from once in two years to once in six years (see Figure 3). The economic consequences of such a change are unevenly distributed. Average net income from lands in the tank command would decline by ~40 percent, and so would employment generated for other households, but the productivity of non-command lands would of course not be affected.
The possibility of negotiating these tradeoffs is further complicated by the fact that the communities that use the forest heavily are generally distinct from those that cultivate in the tank command, and by the lack of a well-defined institution for forest management. The findings in the other dry site (Arepalya) illustrate how the hydrogeological context can combine with the socio-technical context to further complicate the impact of forest cover change. Downstream communities depend upon groundwater and not surface flows. Steep slopes and a rocky geology result in limited infiltration in the catchment, and the main contribution to groundwater comes from percolation from the streambed during flash flooding in the valley. Consequently, changes in catchment land-cover will have limited impact on streamflow and groundwater recharge.
While there has been an apparent decline in groundwater levels in the past decade, and a corresponding increase in groundwater exploitation, the major cause of this decline is still the rainfall pattern; neither pumping nor forest cover change can explain this adequately. In this context, hydrological services per se will not form a significant motivation for any community to get involved in forest conservation.
Overall, our findings indicate that the impact of forest cover change is not so much due to change in evapotranspiration as due to changes in surface soil infiltration properties, and that the manner in which this biophysical impact translates into socio-economic terms depends very much on the geological and technological context—local farmers may or may not face tradeoffs between forest product availability and hydrological service. Further, if any reconciliation of such tradeoffs in a broad-based manner has to take place, institutions to assign forest and water rights clearly and equitably will have to be put in place.