AN EMPIRICAL V-NOTCH WEIR EQUATION AND STANDARD PROCEDURE TO ACCURATELY ESTIMATE DRAINAGE DISCHARGE

Abstract

A reliable empirical flow equation for V-notch weirs will provide flow estimates that can be used to calculate nutrient loads leaving fields with subsurface drainage. The objective of this study was to develop such an equation for an AgriDrain metal-edge sharp-crest 45° V-notch weir. In this undertaking, we measured flow rate with a combination of the weighing method for low flow and a turbine flow meter for high flow. The head of water (H) was measured inside a 25-cm AgriDrain control structure with a three-step method. First, we measured the water level (a) and height of the control structure (b). Second, we measured the height of the V-notch apex (c). Third, we calculated head using this equation: H= (b-a) – (b-c). Based on the flow meter readings (Q) and H measurements, we developed the following stage-discharge equation: Q = 0.749H 2.25, with Q in liters per minute and H in centimeters. This equation is valid for an H less than the height of the V-notch (i.e., flow through the V-notch) with unsubmerged flow. Based on field experience, we provide a standard procedure for accurate estimation of drainage discharge. In conclusion, the stage-discharge equation developed in this study can provide reliable flow estimates for subsurface drainage studies.