The hydrologic sciences and water resources management have long depended on a combination of in situ measurements and remotely sensed data for research and regulatory purposes. In situ measurements are constrained in their spatial distribution by both logistics and costs. Satellite‐ and manned aircraft‐based remote sensing provide spatially broad data, but often of a coarse resolution. Furthermore, all these data collection methods are often limited in their ability to respond to specific short‐term events when the data and imagery could inform real‐time assessment and decision‐making, especially during and immediately after natural disasters. Researchers and regulators are increasingly using unmanned aerial vehicles (UAVs) to fill these gaps in both spatial and temporal data resolution. UAVs are often more cost‐effective than traditional remote sensing or in situ data collection methodologies and the availability of new advancements in UAV design, power supply, payload capacity, and sensors has been driving rapid innovations in their use in the hydrological sciences. This in‐depth review explores these UAV technologies and how they are being employed for on‐demand and cost‐effective characterization, monitoring, assessment, and modeling of soil and water resources. Our discussion addresses both key opportunities and challenges in surface water and groundwater studies, land‐atmosphere interaction studies, water management in agriculture and forestry, ecosystem monitoring, and hydrological modeling.

High-quality bathymetric maps of inland water bodies are a common requirement for hydraulic engineering and hydrological science applications. Remote sensing methods, such as space-borne and airborne multispectral imaging or lidar, have been developed to estimate water depth, but are ineffective for most inland water bodies, because of the attenuation of electromagnetic radiation in water, especially under turbid conditions. Surveys conducted with boats equipped with sonars can retrieve accurate water depths, but are expensive, time-consuming, and unsuitable for unnavigable water bodies. We develop and assess a novel approach to retrieve accurate and high-resolution bathymetry maps. We measured accurate water depths using a tethered floating sonar controlled by an unmanned aerial vehicle (UAV) in a lake and in two different rivers located in Denmark. The developed technique combines the advantages of remote sensing with the potential of bathymetric sonars. UAV surveys can be conducted also in unnavigable, inaccessible, or remote water bodies. The tethered sonar can measure bathymetry with an accuracy of ∼2.1 % of the actual depth for observations up to 35 m, without being significantly affected by water turbidity, bed form, or bed material.