The relationship between runoff rate and lag time and the effects of surface treatments at the plot scale

Rainfall and runoff data collected from plots (108 m(2)) at 1-min intervals allow accurate determination of the time difference between peak rainfall intensity and peak runoff rate. Manning's equation for fully turbulent flows implies that a power function can be used to characterize the relationship between the lag time and peak runoff rate. An exponent close to -0.4 would validate the Manning's equation for overland flow and the coefficient can then be used to estimate the roughness coefficient (Manning's n) during natural storm events. The relationship between the observed lag time and peak runoff rate was investigated for three different surface treatments on a loamy sand: (a) bare fallow, (b) farmers' practice-pineapple beds with bare furrows constructed across the slope, and (c) furrows covered with mulch from residues of previous pineapple crops. Manning's equation was found to be applicable for these treatments and Manning's n is consistently between 0.04 and 0.06. The relationship between lag time and peak runoff rate can be used to model runoff hydrographs at the plot scale. The effect of mulch cover on flow resistance was found to be minimal. Velocity measurements using a dye tracing technique at the same site showed similar results, although the estimated Manning's n was lower. Resistance to flow was significantly increased only when fresh pineapple leaves from a growing pineapple crop were in contact with the soil.