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ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Agroecosystems Management Research » Research » Publications at this Location » Publication #414357

Research Project: Sustainable Intensification in Agricultural Watersheds through Optimized Management and Technology

Location: Agroecosystems Management Research

Title: Calibration of V-notch and compound weirs for subsurface drainage water level control structures

Author
item KATUWAL, SHEELA - Orise Fellow
item JOHNSON, GABRIEL - Iowa State University
item CRAIG, ANDREW - Iowa State University
item Rogovska, Natalia
item ISENHART, THOMAS - Iowa State University
item Malone, Robert - Rob

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/9/2024
Publication Date: 8/16/2024
Citation: Katuwal, S., Johnson, G.M., Craig, A.J., Rogovska, N.P., Isenhart, T.M., Malone, R.W. 2024. Calibration of V-notch and compound weirs for drainage water level control structures. Applied Engineering in Agriculture. 40(4). https://doi.org/10.13031/aea.16032.
DOI: https://doi.org/10.13031/aea.16032

Interpretive Summary: Conservation practices which treat water from artificial subsurface drainage (tile drainage) use a weir water flow control structure to estimate the flow rate of water. Flow rate is calculated by measuring depth of water above the weir and by using relevant weir equation. There is, however, a lack of clarity on how flow rate can be estimated when the depth of water exceeds the top of the weir when a V-shaped weir (V-notch) is used. In this study, weir equations were developed for V-notches based on the size of the control structure to calculate the flow rate of water when the depth of water is either within the V-notch or when it flows over the V-notch. On comparing the newly developed equations with other available methods, flow rates calculated with the new equations resulted in lower errors and the calculated flow rates were closer to the measured flow rates. The results suggest that the new equations will accurately estimate flow rates and will improve the estimation of nutrient loads. These findings will be of interest to conservationist, land managers or farmers, researchers, and engineers involved in estimating/monitoring flow discharge using control structures in edge-of-field conservation practices.

Technical Abstract: Edge-of-field conservation practices use 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 control structures of different sizes for flows within the V-notch and overtopping flow events. The discharge equation for overtopping events (QCW, L s-1) was obtained as: QCW = a1(hb1-h1b1)+a2(W-Wv)h1b2, where h and h1 are heads above vertex/bottom and top of V-notch (cm), respectively, W is the crest width of rectangular weir (cm), Wv is the top width of V-notch (cm), a1 and b1 are parameters for V-notch weir obtained by calibration, and a2 and b2 are calibration parameters for rectangular weir obtained from the literature. The results were compared with a weir equation available in literature (QV+R) which combines V-notch equation with head set equal to V-depth and a rectangular weir equation for flow above V-depth. Discharge at overflow was estimated with high accuracy with QCW whereas QV+R underestimated discharge (e.g., PBIAS of 0.67% vs. 17.82% for a 15.2 cm structure). An example using QV+R resulted in 8.4 % lower annual estimation of nitrate-N load diverted to a saturated buffer than QCW, due to underestimation of drainage discharge during high flow events. The results suggest that the developed equation (QCW) accurately estimates discharge and will thus improve estimated N load compared to QV+R.