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Hydrological Drought Atlas for the State of Texas

D. Rajsekhar, V.P. Singh, A. Mishra

Maps depicting the spatial variation of hydrological drought severity for Texas are presented in this report. The return periods chosen are 5, 10, 25, 50 and 100 years for drought durations of 3, 6, 9, 12, 18, 24 and 36 months, respectively. The maps were constructed using the drought severity-duration-frequency (S-D-F) relationship derived using a copula-based multivariate probabilistic approach. For purposes of deriving drought properties, monthly stream flow simulations from a large scale land surface model, known as variable infiltration Capacity (VIC) model, were utilized. The stream flow time series obtained was gridded at 1/8th degree resolution over Texas. The marginal distribution most suitable for fitting each of the drought variables was determined after testing commonly used distributions, such as exponential, gamma, log-normal, and Weibull. The marginal distributions of drought severity and duration with the smallest root mean square error (RMSE) value between observed and theoretical probabilities were selected. For modeling the joint distribution of drought characteristics, the following classes of bivariate copulas were considered: Archimedean, extreme value, Plackett and elliptical families. The best performing copula, determined using the RMSE and the Akaike information criterion (AIC), were used to determine the conditional and joint return periods and hence derive the drought S-D-F curves. The information obtained from the S-D-F curves was used for the preparation of drought atlas, which depicts the spatial variation of drought severity for specific drought durations and return periods in Texas.

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