Having reliable water monitoring networks is an essential component of water resources and environmental management. A standardized process for the design of water monitoring networks does not exist with the exception of the World Meteorological Organization (WMO) general guidelines about the minimum network density. While one of the major challenges in the design of optimal hydrometric networks has been establishing design objectives, information theory has been successfully adopted to network design problems by providing measures of the information content that can be deliverable from a station or a network. This review firstly summarizes the common entropy terms that have been used in water monitoring network designs. Then, this paper deals with the recent applications of the entropy concept for water monitoring network designs, which are categorized into (1) precipitation; (2) streamflow and water level; (3) water quality; and (4) soil moisture and groundwater networks. The integrated design method for multivariate monitoring networks is also covered. Despite several issues, entropy theory has been well suited to water monitoring network design. However, further work is still required to provide design standards and guidelines for operational use.

The design of optimal hydrometric networks is an important starting point in water resources planning and management. Redundant or inappropriate networks may require unnecessary monitoring costs, while a sparse network may cause a lack of understanding of the process being monitored. Many studies employ information theory, which uses the Shannon entropy, as a measure of the information to design optimal hydrometric networks measuring various hydrologic parameters, such as streamflow and precipitation. The majority of entropy application methods in hydrometric network design have had two common objectives, i.e., maximizing joint entropy and minimizing total correlation. However, it is still unclear what data lengths should be adequate to properly use the entropy approach to network design and how the data lengths affect the entropy values. In this study, four different data lengths (e.g., 5, 10, 15, and 20 years) of daily time series are used to determine the optimal streamflow and precipitation networks using entropy theory coupled with multiobjective optimization. The spatial distributions of the optimal monitoring locations appeared similarly for each data length. Specifically, the hot-spots where the selection likelihood from optimization results is high were not significantly changed; this is more obvious when the data length of daily time series was 10 years or greater. Additionally, the joint entropy and total correlation of the optimal networks were calculated from 10 days to 20 years with a 10-day increment. The joint entropy increased significantly during the first 5 years and then gradually increased without significant change after 10 years. Similarly, the total correlation stabilized after 5 years of daily time series lengths with no major change after 10 years. Therefore, it is recommended to use at least 10 years of data for information theory–based hydrometric network design when using daily time series.

Entropy applications in hydrometric network design problems have been extensively studied in the most recent decade. Although many studies have successfully found the optimal networks, there have been assumptions which could not be logically integrated into their methodology. One of the major assumptions is the uncertainty that can arise from data processing, such as time series simulation for the potential stations, and the necessary data quantization in entropy calculations. This paper introduces a methodology called ensemble-based dual entropy and multiobjective optimization (EnDEMO), which considers uncertainty from the ensemble generation of the input data. The suggested methodology was applied to design hydrometric networks in the Nelson-Churchill River Basin in central Canada. First, the current network was evaluated by transinformation analysis. Then, the optimal networks were explored using the traditional deterministic network design method and the newly proposed ensemble-based method. Result comparison showed that the most frequently selected stations by EnDEMO were fewer and appeared more reliable for practical use. The maps of station selection frequency from both DEMO and EnDEMO allowed us to identify preferential locations for additional stations; however, EnDEMO provided a more robust outcome than the traditional approach.