Assessment of rainfall-runoff characteristics considering temporal variability in Hancheon watershed of Jeju Island

Abstract

Increasing the average rainfall and number of heavy rainfall events created the demand of rainfall-runoff characteristics estimation in Hancheon watershed of Jeju Island. This thesis paper considered the NRCS CN (Natural Resources Conservation Service-curve number), unit hydrograph method for understanding the runoff and developed L-moments method for rainfall frequency analysis. Firstly, the NRCS CN method application result of five years rainfall showed 18% to 44% runoff variation from the total annual rainfall and the 2012 year received maximum rainfall-runoff volume. After that, unit hydrograph method was applied for estimation of peak runoff and peak time. To analyze observed rainfall-runoff and time, four storm events from 2012 were selected for calibration and typhoon Nakri of 2014 was used for validation. The HEC-hydrologic modeling system (HMS) simulation results showed the peak runoff varies from 151 to 546 m3/sec and peak runoff time varies 8 to 27 hour. Meantime, a comprehensive relationship between Clark unit hydrograph parameters (time of concentration and storage coefficient) was also derived. The optimized values of hydrograph parameter were statistically verified by the analysis of variance (ANOVA) and the runoff comparisons were performed by r-square, root mean square error (RMSE) and Nash-Sutcliffe efficiency (NSE) method. The statistical performances of NSE, RMSE and r-square were found as 0.88, 37.52 and 0.76, respectively. The analysis results provide a decision for selecting rainfall-runoff event. Final analysis was on rainfall frequency estimation considering spatio-temporal variability. As a result, L-moments based statistical analysis techniques were shown the dependable results. In the study, the recorded hourly rainfall data series of five rainfall stations were sorted by maximum consecutive hour rainfall (6-, 12-, 24-hour). Then, independence and stationary test of rainfall stations were analyzed using Mann-Kendal and autocorrelation function (ACF) analysis. Hereafter, cluster analysis and discordancy measure showed that the Hancheon watershed is belonging in three regions. Then, L-moments based heterogeneity measure identified that Gumbel and generalized extreme value (GEV) distribution as robust distribution for the study area. Afterwards, Monte Carlo simulation was applied to evaluate the accuracy of frequency estimation and the root mean square error (RMSE). In contrast, the RMSE values for watershed were seen as 0.014 to 0.237 for Gumbel and 0.115 to 0.301 for GEV distribution. The linear regression analysis of the frequency r-square value showed a variation of 0.842 to 0.974. In essence, the assessment of rainfall-runoff characteristics following the above methods can be provided reliable and accurate results. Thus, these study findings are being suggested for water resources planning of Hancheon watershed as well as for Jeju Island.