Approximately 50% of irrigated land is surface irrigated in the U.S.  In contrast to natural rainfall, irrigation is a controlled event, typically with a certain amount of runoff planned to achieve acceptable irrigation uniformity.  Runoff water from surface irrigated fields is often reused on downstream fields within an irrigation tract.  Runoff not captured for reuse is normally discharged to a river or other surface water body.

Surface irrigation runoff carries sediment from fields.  Sediment concentrations of 1,000 to 10,000 mg L^-1 are not uncommon in runoff from recently tilled furrow irrigated silt loam fields with poor management.  Even with good management practices sediment concentrations may still exceed 100 mg L^-1.  The total maximum daily load (TMDL) goal for the middle reach of the Snake River in Idaho is 52 mg L^-1 for suspended sediment.  This goal will be difficult to achieve with return flows from surface irrigation tracts (Table 1).

An irrigation tract can be a sink for soluble or suspended elements as diverted water infiltrates within the tract (Tables 1 & 2), especially if a majority of the land is sprinkler irrigated.  The total P concentration in the irrigation inflow and return flow often exceeds a TMDL goal (0.075 mg L^-1 for Mid-Snake River).  Assuming a sediment total P concentration of 1000 mg kg^-1, the yearly average return flow for the surface irrigated tract shown in (Table 1) would have a total P concentration of about 1.1 mg L^-1.
 
 
 

Table 1.  Soluble Salt Balances - Average Yearly Change and Average Yearly Return Flow Concentrations for a 203,000 acre Irrigation Tract.

Table 2.  Water, Sediment and Phosphorus Balances for Two Southern
Idaho Irrigation Tracts