Time-lapse photography in measurements of surface runoff from sloping farmland: Dynamics of runoff water volumes pooled at the bottom of a sloping field

In recent years, the increased incidence of sudden heavy rains in Japan has enhanced the occurrences of surface runoff from sloping farmlands. These runoff events negatively affect the ecosystems due to leaching of fertilizer components and/or heavy metals into local watersheds. Water-level meters installed at the bottom edge of sloping farmlands have been conventionally used for measuring the extent of surface runoff. However, data provided by this equipment are unreliable because of the direct contact of sensors with various objects, such as pile-ups of crop residues and/or soil particles. To avoid this disadvantage, we herein propose a novel method that uses automatically captured time-lapse images of surface runoff. We devised an automated image-capturing system and have been using it in an outdoor artificial sloping field in Ibaraki Prefecture, Japan for over three months. Several sets of captured images, recorded actual rainfall levels, and runoff amounts were obtained during five typical runoff events caused by natural rainfalls. The captured images were analyzed at 10-minute intervals to determine the total volumes of water pools that were eventually formed with accumulated runoff water at the bottom of the sloping experimental field during each event. The estimates of accumulated water volume reasonably correlated with the instantaneous rainfall levels (r = 0.79sup(***)). Calculations were done for eight intense rainfall peaks during the observation period, assuming that the actual runoff amount could be estimated as the sum of the water amount measured with the water-level meter and the amount of accumulated water increased with time. We found that ratios of the amounts of surface runoff to rainfall water in each of the 10-minute intervals were within the range 12-59%. Our results are consistent with those of a previously published report of an experiment conducted at the same site but with a different method. The proposed method may provide more precise estimates of the amount of surface runoff during intense rainfall events.