Using the Soil Conservation Service (SCS) curve number (CN) procedure for estimating runoff volume on an ungauged forest watershed remains controversial because little guidance has been provided for defining appropriate CN values. In this study, alternative methods for assigning CN values (CNs) were assessed to determine whether these methods provide acceptable estimates of runoff on forested watersheds. The estimated CNs varied between the methods employed, showing the highest CN values when derived from a probabilistic method and lowest when derived from a graphical method. The tabulated CN values in Section 4 of the National Engineering Handbook (NEH-4) had relatively higher bias compared to those derived from measured rainfall-runoff data. The storm runoff volume was predicted using the assigned CNs and compared with the observations. The coefficients of determination and RMSE values between the measured and estimated runoff volumes varied with the methods employed. The highest watershed average RMSE value was obtained by the use of the tabulated CN values in NEH-4 (51.19 mm), while arithmetic mean approach provided the lowest average RMSE value of 24.38 mm, even though this method requires intensive data collection. Among the alternatives, probabilistic method was found to be the most reliable in determining CNs for forest cover with limited data. The estimated runoff largely agreed with the observations. Therefore, the revised CNs can be used for estimating storm runoff from ungauged, mountainous forests.
The water retention capacity of forest leaf litter was estimated through lysimeter measurements under field conditions. Six lysimeters were placed in Pinus koraiensis and Quercus acutissima forests and filled with the surrounding leaf litter to represent the effects of litter type on the water retention capacity. Two years of measurements for rainfall and litter weight have been conducted in all lysimeters at 30 min intervals. Field measurements showed that P. koraiensis litter stored more water during rainfall periods than did Q. acutissima litter. As a result, immediately after the cessation of rainfall, 1.82 mm and 3.00 mm of water were retained per unit mass of Q. acutissima and P. koraiensis litter, respectively. Following rainfall, after the gravitational flow had entirely drained, the remaining water adhered to the litter was estimated to be 1.66 ± 1.72 mm and 2.72 ± 2.82 mm per unit mass per rainfall event for Q. acutissima and P. koraiensis litter, respectively. During the study period, approximately 83.7% of incident rainfall drained into the uppermost soil layer below the Q. acutissima litter, whereas 84.5% of rainfall percolated through the P. koraiensis litter. The moisture depletion curves indicated that 50% of the water retained in the Q. acutissima and P. koraiensis litter was lost via evaporation within 27 h and 90 h after the cessation of rainfall, respectively. This study demonstrated the water retention storage of leaf litter and its contribution to the water balance over floor litter according to litter and rainfall characteristics. The results also proved that lysimetry is a reliable method to quantify the variation of litter moisture under natural conditions.
Fires can alter the hydraulic properties of burned soils through the consumption of organic matter on the ground surface. This study examined the effects of rainfall on the presence of soil pore clogging with varying ash layer thickness using laboratory rainfall simulator experiments. The image analysis with resin impregnation showed that rainfall impact caused plugging of soil pores at 22.2% with soil particles and 14.3% with ash particles on near surface soils (0–5 mm below). High rainfall intensities enhanced soil pore clogging by ash particles, particularly at shallow soil depths (0–10 mm). Ash deposits on the soil surface increased the water-absorbing capacity of ash-covered soils compared with that of bare soils. The rainfall simulation experiments also showed that ash cover led to a reduction in soil hydraulic conductivity, owing to the combined effects of surface crust formation and soil pore clogging. The complementary effects of soil pore clogging and water absorption by ash cover could hamper the accurate understanding of the soil hydrologic processes in burned soils.
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