Seepage from Channeled Flows as Influenced by PAM and Sediment.

Seepage from water streams into unlined channels determines the proportion of water distributed to adjacent soil for plant use or soil or groundwater recharge, or conveyed to downstream reaches.  We conducted a laboratory study to determine how sediment type (none, clay, silt), sediment concentration (0, 0.5, and 2 g L^-1), and water soluble, anionic polyacrylamide (PAM) concentration (0, 0.4, 2 mg L^-1) influences seepage loss of irrigation water (electrical conductivity = 0.04 Sm^-1; sodium adsorption ratio = 2.2) from unlined channels in silt loam soil.  In a miniflume a preformed channel with 7% slope was supplied with 40 mL min^-1 simulated irrigation water inflows containing the different treatment combinations.   Runoff and seepage rates and runoff sediment were monitored for 24-h.  Average 23-h cumulative seepage loss was 11.8 L for silt-loaded inflows, 2.8 L for Clay loaded inflows, and 6.4 L for flows without sediment.  Increasing inflow clay concentrations, 0, 0.5, 2 g L^-1 clay, decreased cumulative seepage volume (23-h) for the No-PAM treatment from 12.4 to 6.7, and 0.2 L respectively. Increasing inflow silt concentrations in No-PAM treatments resulted in a curvilinear response with a seepage volume maximum occurring for the 0.5-g L^-1 treatment (12.4, 47.1, 9.8 L respectively).  Increasing inflow PAM concentrations increased seepage volumes for 2-g L^-1 Silt and 2-g L^-1 Clay treatments, but decreased seepage for the 0.5-g L^-1 Silt treatment.    Seepage losses from these unlined channels can be significantly altered relative to untreated controls by manipulating the sediment particle size and concentration, and PAM concentration of irrigation water inflows.  Their effects on induced seepage changes are complex, strongly controlled by factor interactions, and appear to involve a number of mechanisms.