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DROUGHT CHARACTERIZATION USING PROBABILISTIC MODELS
Droughts are complex natural disasters caused due to deficit in water availability over a region. Water availability is strongly linked to precipitation in many parts of the world that rely on monsoonal rains. Recent studies indicate that the choice of precipitation datasets and drought indices could influence drought analysis. Therefore, drought characteristics for the Indian monsoon region were reassessed for the period 1901-2004 using two different datasets and standard precipitation index (SPI), standardized precipitation-evapotranspiration index (SPEI), Gaussian mixture model-based drought index (GMM-DI), and hidden Markov model-based drought index (HMM-DI). Drought trends and variability were analyzed for three epochs: 1901-1935, 1936-1970 and 1971-2004. Irrespective of the dataset and methodology used, the results indicate an increasing trend in drought severity and frequency during the recent decades (1971-2004). Droughts are becoming more regional and are showing a general shift to the agriculturally important coastal south-India, central Maharashtra, and Indo‑Gangetic plains indicating food security challenges and socioeconomic vulnerability in the region.
Drought severities are commonly reported using drought classes obtained by assigning pre-defined thresholds on drought indices. Current drought classification methods ignore modeling uncertainties and provide discrete drought classification. However, the users of drought classification are often interested in knowing inherent uncertainties in classification so that they can make informed decisions. A probabilistic Gamma mixture model (Gamma-MM)-based drought index is proposed as an alternative to deterministic classification by SPI. The Bayesian framework of the proposed model avoids over-specification and overfitting by choosing the optimum number of mixture components required to model the data - a problem that is often encountered in other probabilistic drought indices (e.g., HMM-DI). When sufficient number of components are used in Gamma-MM, it can provide a good approximation to any continuous distribution in the range (0,infinity), thus addressing the problem of choosing an appropriate distribution for SPI analysis. The Gamma-MM propagates model uncertainties to drought classification. The method is tested on rainfall data over India. A comparison of the results with standard SPI shows significant differences, particularly when SPI assumptions on data distribution are violated.
Finding regions with similar drought characteristics is useful for policy-makers and water resources planners in the optimal allocation of resources, developing drought management plans, and taking timely actions to mitigate the negative impacts during droughts. Drought characteristics such as intensity, frequency, and duration, along with land-use and geographic information, were used as input features for clustering algorithms. Three methods, namely, (i) a Bayesian graph cuts algorithm that combines the Gaussian mixture model (GMM) and Markov random fields (MRF), (ii) k-means, and (iii) hierarchical agglomerative clustering algorithm were used to find homogeneous drought regions that are spatially contiguous and possess similar drought characteristics. The number of homogeneous clusters and their shape was found to be sensitive to the choice of the drought index, the time window of drought, period of analysis, dimensionality of input datasets, clustering method, and model parameters of clustering algorithms. Regionalization for different epochs provided useful insight into the space-time evolution of homogeneous drought regions over the study area. Strategies to combine the results from multiple clustering methods were presented. These results can help policy-makers and water resources planners in the optimal allocation of resources, developing drought management plans, and taking timely actions to mitigate the negative impacts during droughts.