Most of the sediment detached on surface irrigated fields with uniform slope occurs on the upper quarter or third of the field.  Detached sediment either leaves the field with runoff or is deposited on the lower end of the field when transport capacity decreases as water infiltrates.  This moves topsoil from the upper end to the lower end or completely off the field. Topsoil depth affects crop productivity.  It is estimated that over 50% of the fields in one surface irrigated tract suffer some yield losses from past soil erosion.
 
 
 

Most soil is eroded on the inflow-end of furrow irrigated fields.	Soil erosion during 80 years of surface irrigation has exposed white subsoil.