Studying reach-scale spatial hydrology in ungauged catchments

- New approach directly maps spatial hydrology in ungauged, large rivers.
- Combines MODIS and topographic data to map spatial hydrology.
- “Virtual” gauging stations measure and map hydrologic metrics at reach-scale.
- We map flood flow, pulse shape, duration, amplitude and connectivity.

SummaryDryland regions are home to some of the most poorly gauged rivers on Earth. Consequently, these regions lack a detailed understanding of the hydrology, are associated with underdevelopment and significant socio-economic disadvantage, though there is increasing pressure to develop the water resources in these areas. However, this is often limited by a lack of data from which to understand regional hydrology and water-dependent processes and make informed water resource management decisions. This paper presents a novel approach to directly map, from remotely sensed imagery, the five flow types and six hydrological metrics defined as the most significant determinants of ecological condition of dryland rivers (Flow (duration of flow), Amplitude (last maximum depth), Pulse Shape (duration of rising limb and falling limb), Duration (present length of inundation), Connection (duration of present downstream connection)).At fourteen “virtual” gauging stations in two rivers (Newcastle Creek and Playford River) on the Barkly Tablelands, northern Australia, daily classified Moderate Resolution Imaging Spectroradiometer (MODIS) imagery was used to map flow types and metrics at 250 m resolution, across 1996 km2. The performance of the “virtual” gauging stations is validated against traditionally gauged data at two locations and confirms that hydrological data significant for ecology can be extracted from daily flood mapping using remotely sensed MODIS imagery. Results found a pronounced downstream trend in flow characteristics from more ephemeral uplands to seasonally-inundated lowlands. Significant between reach variability in the Duration and Connection is also noted, which is related to cross section morphology and river position. It is suggested that this approach could be applied to other poorly or ungauged large, dryland rivers, where the requirements, including limited cloud cover, long (weeks to months) flood pulses, inundation widths of greater than 2 km, and lateral floodplain gradients of less than approximately 0.005 m/m are met. This novel approach for the measurement of ungauged basins offers significant potential to allow research relevant to hydrology and water-dependent processes where traditional approaches to dryland river hydrology are limited by the lack of gauging infrastructure, or by complex multi-channel and low-gradient geomorphology.