Rational Method

It is considered the most widely employed and easy-to-use practical method to estimate and determine the surface runoff of any specific rainfall. This method is used primarily on small scales. The rational approach is expressed mathematically as

$$\mathbf{Q} = \mathbf{C} \mathbf{I} \mathbf{A} \tag{5}$$

where C = coefficient of runoff (runoff volume/rainfall), A = area of the catchment, and I = intensity of the precipitation.

We made sizeable holes and installed plastic containers with rigid channels that channel water into the collection bottles at the bottom of each plot to measure runoff volume (Figure 4). The surface runoff volume of the plants was collected in this plastic container at the same rainfall intensity and duration during the rainfall events and transferred to a measuring cylinder to ge<sup>t</sup> the readings. The surface runoff volume of each vegetation was therefore determined by dividing volume by the period.

**Figure 4.** Experimental surface runoff plot.

This experiment shows how vegetation characteristics such as the vegetation roots and vegetation ground cover reduce surface runoff. This experiment only considers surface runoff from September to December 2020. We collected four rainfall events for this study. These event dates were chosen based on a weather forecast predicting rainfall of >12 mm, which matched the experimental criteria. The daily rainfall data was obtained from the National Hydraulic Research Institute of Malaysia (NAHRIM).
