In accordance with the rapid proliferation of machine learning (ML) and data management, ML applications have evolved to encompass all engineering disciplines. Owing to the importance of the world’s water supply throughout the rest of this century, much research has been concentrated on the application of ML strategies to integrated water resources management (WRM).
1. Introduction
In recent years, machine learning (ML) applications in water resources management (WRM) have garnered significant interest
[1]. The advent of big data has substantially enhanced the ability of hydrologists to address existing challenges and encouraged novel applications of ML. The global data sphere is expected to reach 175 zettabytes by 2025
[2]. The availability of this large amount of data is forming a new era in the field of WRM. The next step for hydrological sciences is determining a method to integrate traditional physical-based hydrology into new machine-aided techniques to draw information directly from big data. An extensive range of decisions, from superficial to complicated scientific problems, is now handled by various ML techniques. Only a machine is capable of fully utilizing big data because of its veracity, velocity, volume, and variety. In recent decades, ML has attracted a great deal of attention from hydrologists and has been widely applied to a variety of fields because of its ability to manage complex environments.
In the coming decades, the issues surrounding climate change, increasing constraints on water resources, population growth, and natural hazards will force hydrologists worldwide to adapt and develop strategies to maintain security related to WRM. The Intergovernmental Hydrological Programme (IHP) just started its ninth phase plans (IHP-IX, 2022-2029), which place hydrologists, scholars, and policymakers on the frontlines of action to ensure a water-secure world despite climate change, with the goal of creating a new and sustainable water culture
[3]. Moreover, the rapid growth in the availability of hydrologic data repositories, alongside advanced ML models, offers new opportunities for improved assessments in the field of hydrology by simplifying the existing complexity. For instance, it is possible to switch from traditional single-step prediction to multi-step ahead prediction, from short-term to long-term prediction, from deterministic models to their probabilistic counterparts, from univariate to multivariate models, from the application of structured data to volumetric and unstructured data, and from spatial to spatio-temporal and the more advanced geo-spatiotemporal environment. Moreover, ML models have contributed to optimal decision-making in WRM by efficiently modeling the nonlinear, erratic, and stochastic behaviors of natural hydrological phenomena. Furthermore, when solving complicated models, ML techniques can dramatically reduce the computational cost, which allows decision-makers to switch from physical-based models to ML models for cumbersome problems. Therefore, the new emerging hydrological crises, such as droughts and floods, can now be efficiently investigated and mitigated with the assistance of the advancements in ML algorithms.
2. Major Application of ML in WRM
ML algorithms are typically categorized into three main groups: supervised, unsupervised, and RL
[4]. A comparison of these is summarized in
Table 1. Supervised learning algorithms employ labeled datasets to train the algorithms to classify or predict the output, where both the input and output values are known beforehand. Unsupervised learning algorithms are trained using unlabeled datasets for clustering. These algorithms discover hidden patterns or data groupings without the need for human intervention. RL is an area of ML that concerns how intelligent an agent is to take action in an environment to obtain the maximum reward. In both supervised and RL, inputs and outputs are mapped such that the agent is informed of the best strategy to take in order to complete a task. In RL, positive and negative behaviors are signaled through incentives and penalties, respectively. As a result, in supervised learning, a machine learns the behavior and characteristics of labeled datasets, detects patterns in unsupervised learning, and explores the environment without any prior training in RL algorithms. Thus, an appropriate category of ML is required based on the engineering application. The major ML learning types in WRM are summarized in
Figure 1, where the first segment covers the core contents of the research reviewed in the following sections.
Figure 1. Four major types of machine learning.
Table 1. Comparison of supervised, unsupervised, and reinforcement learning algorithms.