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Understanding the nature of climatic change impacts on spatial and temporal hydroclimatic patterns is important to the development of timely and spatially explicit adaptation options. However, regime-switching behavior of hydroclimatic variables complicates the modelling process as many traditional time series methods do not capture this behavior. Accurately representing spatial correlation across hydroclimatic regimes is particularly important for water resources planning in large watersheds such as the Colorado River, and regions where interbasin transfers and shared demand nodes link multiple watersheds. Here, we developed a hidden Markov model (HMM) with covariates that generates an ensemble of plausible future regional scenarios of the Palmer modified drought index (PMDI) for any projected temperature sequence. The resulting spatially explicit scenarios represent the historical spatial and temporal patterns of the training data while incorporating non-stationarity by conditioning on temperature. These ensembles can aid water resources managers, infrastructure planners, and government policymakers tasked with building of more resilient water systems. Moreover, these ensembles can be used to generate streamflow ensembles, which, in turn, will be a valuable input to study the impact of climate change on regional hydrology. 

This repository contains R scripts for implementing a computationally efficient 4-state Hidden Markov Model (HMM) that uses temperature as a covariate to generate ensembles of plausible Palmer Modified Drought Index (PMDI) scenarios across the Western U.S. The model uses paleo PMDI data, which spans from 1500 to 1980 with a matrix grid of 1823 x 481 (e.g., 1823 grid-cells and 481 years). Similarly, paleo temperature data covers the same period, arranged in a matrix grid of 1637 grid cells by 481 years. To address the high dimensionality of the datasets, Principal Component Analysis (PCA) is applied to each variable, and the first six principal components (PCs) from both PMDI and temperature are retained as input to the HMM. The trained HMM is then used to simulate future PMDI scenarios by leveraging bias-corrected CMIP6 temperature projections under the Shared Socioeconomic Pathway (SSP) 2–4.5 scenario. The HMM framework is designed to capture the spatiotemporal variability and regime-shifting behavior of hydroclimatic patterns. It provides critical insights into the spatial correlation of wet and dry conditions across the Western U.S., supporting regional drought risk assessment and long-term water resource planning. For a more detailed description of the model, please refer to the following paper: Tezcan, B., & Garcia, M. (2025). Training a hidden Markov model with PMDI and temperature to create climate informed scenarios. Frontiers in Water, 7, Article 1472695. https://doi.org/10.3389/frwa.2025.1472695