Agriculture, water, and ecosystems: avoiding the costs of going too far
plantations for carbon sequestration are new driving forces in the agricultural sector with potentially major, but largely unassessed, consequences for water use (Jackson and others 2005; Berndes 2002). Forest clearing for agriculture has hydrological consequences [well established], but site-specific responses will vary. Deforestation can lead to land degrada- tion through salinization, soil loss, and waterlogging (for discussion on irrigation-induced salinity, see chapter 15 on land).
ere is increasing speculation about how altered green water flows affect local, re-
gional, and global climate. Most of the evidence comes from tropical semiarid to humid climates, with little from temperate regions. is section reviews the evidence of water- related changes in terrestrial ecosystems as a response to agriculture.
Changes in the water table. Water can build up in the soil profile if the rate of input, through irrigation, for example, exceeds the rate of throughput (for example, crop water consumption). is can cause water logging and salinization, which are extensively described for irrigated agriculture (Postel 1999). Continuous irrigation can result in soil salinization. Tanzania has an estimated 1.7–2.9 million hectares of saline soils and 300,000–700,000 hectares of sodic soils (FAO and UNESCO 2003), some of it now abandoned. Salt-affected soils in irrigation schemes are often related to poor soil and water management in addition to the unsuitability of many soils for irrigation (see chapters 9 on irrigation and 15 on land).
Clearing woody vegetation for pastures and crops can also lead to dryland salini- zation. Tree-covered landscape can provide an important regulating service by consum- ing rainfall by high evapotranspiration, limiting groundwater recharge, and keeping the groundwater low enough to prevent salt from being carried upward through the soil. Aus- tralia has had major problems with soil salinization since native woody vegetation was cleared in the 1930s for pastures and agricultural expansion (Farrington and Salama 1996). Consumptive water use has declined, the water table has risen, and salt has moved into the surface soils so that large tracts of land have become less suitable—and even unusable—for agriculture (Anderies and others 2001; Briggs and Taws 2003). Green water flows at a continental scale have been reduced by 10% (Gordon, Dunlop, and Foran 2003).
Decreased infiltration of water into the soil, often as a result of poor management of crop and grazing land, is another problem that can cause changes in the water table with effects on terrestrial systems, including a reduction in the capacity to produce biomass (Falkenmark and Rockström 1993). is is a well-known problem in many rainfed farm- ing systems (see chapters 8 on rainfed agriculture and 15 on land). Desiccation of the soil in this manner is one of the factors behind what is often referred to as “desertification” or land degradation in the tropics.
Changes in runoff from vegetation change. e effects of alterations to vegetation (espe- cially forests) on blue and green water flows are well studied at a local and regional scale. Catchment-scale experiments have shown that forested catchments in general have a high- er green water flow and a lower blue water flow than grass or crop-dominated catchments with the same hydrology and climate. However, the effects of deforestation depend on the intensity and manner of forest clearance and on the character of the old and new land
Decreased infiltration of water into the soil, often a result of poor management of crop and grazing land, can cause desiccation of the soil