**1.1 Water monitoring**

Today water is the most precious and valuable resource. Water quality monitoring can be expensive to carry out and challenging in hard-to-reach locations. Typically, it still requires a grab or spot water sample to be manually taken. This is both costly, time-consuming, and prone to error. It is not possible to achieve good spatial and temporal data for the sites of interest using this approach. The rationale of water monitoring comes from the relevant policy or legislation in each country. The function of water monitoring is to ensure good ecosystem health or provide a water supply suitable for use. Considerations for any water monitoring programme are summarized in **Figure 1**.

Drinking water utilities face new challenges in day-to-day operation due to growing population, ageing infrastructure and climate-related weather events. Therefore there is a need to address more suitable approaches to monitoring water quality. Although the

#### **Figure 1.**

*Considerations when planning a water monitoring programme.*

current monitoring approaches include the determination of physical, chemical and biological species, there are drawbacks: a) poor temporal and spatial information b) laborious and costly approach and c) absence of real-time water quality information to. This highlights the opportunity for continuous water quality monitoring. A drawback of course to using in-situ sensing is the level of maintenance required. In-situ sensing is commonly used for the measurement of common water quality parameters that contribute to and affect the environment. *In-situ* sensors allow for high-resolution monitoring, they can be used alone or deployed as part of an observation system [1, 2]. However, a major limitation currently is the lack of available sensors that can measure priority parameters such as chemicals of emerging concern, bacteria, or natural toxins in water. The benefit of being able to measure such parameters in real-time could mean that effective decision supports could be implemented, and appropriate measures applied.

#### **1.2 New modes of sensing**

For water monitoring many forms of data gathering are emerging including satellites with sensors, aerial vehicles that carry imaging systems and in-situ sensors for direct water quality measurements.

#### *1.2.1 Remote sensing*

Remote monitoring, via satellite assets and distributed in-situ sensors, has the potential to meet the challenges of data gaps in many areas including developing countries. Existing and new satellites are proving useful in coastal waters, estuaries, lakes, and reservoirs, which are relevant to water quality managers. The information from satellites can assist the policy design and implementation.

Chlorophyll a (*Chl-a*) determination in water bodies is a common application of remote sensing for water quality monitoring as an indicator for harmful algal blooms (HAB). Although remote sensing techniques contribute to very promising outputs and offers a replacement method for field sampling, enabling fast, temporal, spatial and frequent observations [3, 4], it suffers from some limitations concerning the accuracy of the products obtained, data continuity, excess tools, and software for atmospheric correction (scattering and absorption effects, cloud cover etc.) and the

precision of the results [4, 5]. Hence the added valve of combining remote sensing with another monitoring tool such as *in-situ* sensing devices.

*In-situ* instrumentation, including flow meters and water quality sensors installed in remote and hard to access locations, can complement satellite information. Remote sensing data is generally used to monitor parameters such as secchi disk depth, temperature, total organic carbon, total suspended matter, turbidity, conductivity, colour dissolved organic matter, *Chl-a*, suspended solids and sea surface salinity [2]. Geographic Information System (GIS) is commonly used alongside remote sensing techniques, it is a tool that is used to process and analyse the changes observed in satellite imagery.
