Irrigation can transform agricultural systems from single-season, rainfed cropping with livestock subsisting on degraded grazing land, to multiple cropping of irrigated land, cash crops, and intensive livestock production (such as stall-fed dairy cattle). Watershed management technologies must be appropriate to local resource conditions, population densities, crop types, and institutional capacities, and they must ensure that poor people benefit (box 5.13). Social institutions. At the community and national levels, watershed management improvements rely on the development of social institutions for the resource user groups for water, forests, and land, as these groups are key to most watershed programs. The groups also represent social capital formation that can have wide applicability in empowering rural people, enabling them to undertake additional development activities. Such groups must often deal with problems of unequal distribution of benefits in which downstream land users benefit at the expense of upstream land users. Box 5.12: Are forests and reforestation beneficial for hydrology and groundwater recharge? Do forests increase runoff? Catchment studies show that because of increased interception, transpiration, and deeper rooting depth in forests compared with cropland or grassland, annual runoff is generally decreased under forests.
Do forests regulate flows? Increased dry season transpiration but increased infiltration and, for cloud forests, cloud water deposition, may augment dry season flows. More and more evidence from catchments worldwide shows that most forests reduce dry season flows. Infiltration properties are critical in partitioning runoff. Effects are site-specific, so more research is needed.
Do forests reduce erosion? Natural forest is associated with high infiltration rates and low soil erosion, but plantations may not show these benefits because of roads, ditches, and splash erosion. Forest canopies may not protect soil from raindrop impacts. More research is needed on species and drop size.
Do forests reduce floods? Canopy interception of rainfall and increased evapotranspiration may reduce floods. However, forest management activities (roads, drains, soil compaction) may increase floods. Studies show flood prevention benefits for small events only in small catchments and little or negative benefit for large rainfall events. Studies in large catchments show no measurable effects on the frequency or magnitude of flooding.
Do forests improve water quality? In general, forest water is of better quality than catchments under grazing or agriculture. Forest management rather than the presence of forests is critical for water quality. In environments where pollution is high, forest catchments and forest water may become acidified.
Source: Calder 1998 |
    
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