Measuring flow velocity under straw mulch using the improved electrolyte tracer method

Most conventional methods cannot measure flow velocities under a canopy without disturbing it. Use of mulch that reduces runoff and soil losses is a common land management practice. It is useful to study flow velocity beneath the mulch canopy to understand the processes involved. An improved method to measure flow velocity that uses an electrolyte tracer was proposed by Lei et al. (2010, J. Hydrology 390, 45–56). This study was designed to illustrate the application of that method by measuring flow velocity under wheat straw mulch. Tap water at flow rates of 2, 4 or 8 L min−1 entered the upper end of flumes (1 m long, 0.25 m wide) set at three slope gradients (5°, 10° or 15°), which contained soil with or without a mulch cover (0.4 kg m−2). Flow velocity was measured at three different distances from the electrolyte injector. The results obtained were qualitatively as expected. In all cases, the mean flow velocity was significantly lower under the mulch than over the bare soil. The flow velocity over the bare soil was found to be on average 23% higher than that under the mulch regardless of the slope gradient or the flow rate. However, flow velocity was significantly affected by the slope gradient and flow rate. The diameter of the sensors (about 4 mm) meant that flow velocity could be measured with minimal disturbance of the mulch, thereby reducing edge effects that can affect the water flow. Therefore, the improved electrolyte tracer method was found to be suitable for conditions where overland flow cannot be observed directly. Thus, the method can be applied in the field to study flow velocity distributions under canopies.