CALIBRATION OF V-NOTCH AND COMPOUND WEIRS FOR SUBSURFACE DRAINAGE WATER LEVEL CONTROL STRUCTURES

Numerous edge-of-field conservation practices use subsurface drainage water level control structures to monitor water levels and estimate discharge. In a control structure, procedures for calculating discharge when flow depth (head) exceeds the V-notch depth and overflows in the rectangular portion of the compound weir (CW) are ambiguous. In this study, we developed calibration equations for V-notch weirs in Agri Drain inline water level control structures of different sizes for flows within the V-notch and overtopping flow events. The discharge equation for overtopping events (QCW, CW , L s-1)-1 ) was determined as: Q CW = a1(hb1-h1b1)+a2(We-Wv)h1b2, 1 (h b1-h 1 b1 )+a 2 (W e-W v )h 1 b2 , where h and h1 1 are heads above vertex/bottom and top of V-notch (cm), respectively, W is the effective crest width of rectangular weir (cm), Wv v is the top width of V-notch (cm), a1 1 and b1 1 are parameters for V-notch weir obtained by calibration, and a2 2 and b2 2 are calibration parameters for rectangular weir obtained from literature. Results were compared with a weir equation available in the literature (QV+R), V+R ), which combines a V-notch equation with a head equal to V-depth and a rectangular weir equation for flow above V-depth. Discharge at overflow was estimated with high accuracy with Q CW, whereas Q V+R underestimated discharge (e.g., PBIAS of 0.67% vs. 17.82% for a 15.2 cm structure). An example using Q V+R resulted in a 14% lower annual estimation of nitrate-N load diverted to a saturated buffer than Q CW due to underestimation of drainage discharge during overflow events. Results suggest that the developed equation (QCW) CW ) accurately estimates discharge and will thus improve the estimated N load compared to Q V+R .