Effects of Agriculture on Hydrology

The hydrologic effects of agricultural land uses depends on the level of soil compaction, the proximity of the disturbance to stream channels, and the proportion of watershed disturbed. Annual row crops in particular expose large areas of soil. A compacted soil layer may develop below the plow zone, restricting water infiltration and increasing surface runoff. As a result, peak flow rates from watersheds with extensive row cropping are greater than from forested watersheds. The lower evapotranspiration of row crops results in a total water yield that is also higher than that with forest cover (SAMAB 1996).

Where wetlands are converted to agriculture, land is drained by the construction of ditches or tile-and-drain systems. Streams that are channelized by straightening and dredging often suffer from channel incision, floodplain isolation, and headward erosion of stream channels. Channelization and wetland drainage can increase the magnitude of flooding, because reduction in storage capacity in upstream portions of a basin results in accelerated water delivery downstream (Poff and others 1997).

Pasture effects on hydrology depend on how the land is managed. Pastures that are overgrazed, have compacted soils, and allow animals free access to riparian areas degrade the downstream system. Overland runoff occurs during storms, resulting in increased peak flows and associated water quality problems. Livestock use often is concentrated in riparian areas because the terrain is flatter, and water, shade, and succulent vegetation are more readily available than in upland areas. As a result, riparian areas may be seriously damaged. Stream banks are often destabilized by trampling and suffer greatly accelerated erosion where livestock have access. Animal feedlots concentrate impacts and accentuate the hydrologic and water-quality problems. Pastures managed for good ground cover, high infiltration rates, and protected stream banks will have fewer stream impacts. Solar-powered pumps can withdraw streamwater to watering troughs to keep livestock out of stream channels (SAMAB 1996).

The extent and magnitude of effects from orchards depend primarily on the amount of bare soil and extent of road development. Soils in well-established orchards should have high infiltration rates. However, the road system may produce rapid surface runoff (SAMAB 1996).