Irrigation-induced erosion

Irrigation-induced erosion is one of the most serious sustainability issues in agriculture, impacting not only the future strategic and commercial viability of irrigation agriculture, but also the survival and comfort of earth's human population. Preventing irrigation-induced erosion to maintain the high crop yield and quality advantages of irrigated agriculture is also a key to the preservation of natural ecosystems. This is because replacement of irrigated production requires two to three times the equivalent rainfed production area to match any lost irrigated production (Sojka, 1998)

The Current State of Predicting Furrow Irrigation Erosion

There continues to be a need to predict furrow irrigation erosion to estimate on- and off-site impacts of irrigation management. The objective of this paper is to review the current state of furrow erosion prediction technology considering four models: SISL, WEPP, WinSRFR and APEX. SISL is an empirical model for predicting annual soil loss from furrow irrigated fields. SISL could potentially be a useful model if a new method was developed to calculate base soil loss for areas other than southern Idaho where it was developed. The WEPP model uses physically-based equations to predict erosion in irrigation furrows, which are assumed to be the same as rills. Primary difficulties with the WEPP model are defining erodibility parameters for furrow irrigation and over-prediction of transport capacity. WinSRFR provides detailed evaluation of furrow hydraulics and sediment detachment, transport and deposition in an individual furrow during a single irrigation event using similar equations as WEPP. Initial evaluations of WinSRFR are promising and development continues to fully simulate the mix of aggregate sizes found in furrow soil and furrow flow. The APEX model uses empirical relationships to predict soil loss from small watersheds. Preliminary evaluation of the APEX model indicated reasonable correlation with measured soil loss in a 170 ha irrigated watershed. All of these methods require further development and/or evaluation before they can be widely applied to furrow irrigated land. In selecting a predictive tool, it should be noted that an empirical equation may be as good as a physically based equation if we cannot quantify the parameters for the physically based equation

Sediment and phosphorus transport in irrigation furrows

Sediment and phosphorus (P) in agricultural runoff can impair water quality in streams, lakes, and rivers. We studied the factors affecting P transfer and transport in irrigated furrows in six freshly tilled fallow fields, 110 to 180 m long with 0.007 to 0.012 m m' slopes without the interference of raindrops or sheet flow that occur during natural or simulated rain. The soil on all fields was Portneuf silt loam (coarse-silty, mixed, superactive, mesic Durinodic Xeric Haplocalcids). Flow rate, sediment concentration, and P concentrations were monitored at four, equally spaced locations in each furrow. Flow rate decreased with distance down the furrow as water infiltrated. Sediment concentration varied with distance and time with no set pattern. Total P concentrations related directly to sediment concentrations (r2 = 0.75) because typically >90% of the transported P was particulate P, emphasizing the need to control erosion to reduce P loss. Dissolved reactive phosphorus (DRP) concentrations decreased with time at a specific furrow site but increased with distance down the furrow as contact time with soil and suspended sediment increased. The DRP concentration correlated better with sediment concentration than extractable furrow soil P concentration. However, suspended sediment concentration tended to not affect DRP concentration later in the irrigation (>2 h). These results indicate that the effects of soil P can be overshadowed by differences in flow hydraulics, suspended sediment loads, and non-equilibrium conditions

Flow in Open Channel with Complex Solid Boundary

yesA two-dimensional steady potential flow theory is applied to calculate the flow in an open channel with complex solid boundaries. The boundary integral equations for the problem under investigation are first derived in an auxiliary plane by taking the Cauchy integral principal values. To overcome the difficulties of a nonlinear curvilinear solid boundary character and free water surface not being known a priori, the boundary integral equations are transformed to the physical plane by substituting the integral variables. As such, the proposed approach has the following advantages: (1) the angle of the curvilinear solid boundary as well as the location of free water surface (initially assumed) is a known function of coordinates in physical plane; and (2) the meshes can be flexibly assigned on the solid and free water surface boundaries along which the integration is performed. This avoids the difficulty of the traditional potential flow theory, which seeks a function to conformally map the geometry in physical plane onto an auxiliary plane. Furthermore, rough bed friction-induced energy loss is estimated using the Darcy-Weisbach equation and is solved together with the boundary integral equations using the proposed iterative method. The method has no stringent requirement for initial free-water surface position, while traditional potential flow methods usually have strict requirement for the initial free-surface profiles to ensure that the numerical computation is stable and convergent. Several typical open-channel flows have been calculated with high accuracy and limited computational time, indicating that the proposed method has general suitability for open-channel flows with complex geometry