M. Andreini

Determining watershed response in data poor environments with remotely sensed small reservoirs as runoff gauges

In the semiarid regions of Africa, there are many small reservoirs used for irrigation. This study explores the practicality of using small reservoirs as runoff gauges by estimating their water storage changes using remote sensing imagery. A simple rainfall-runoff model is developed by observing the surface area and estimating the volume of eight small reservoirs in the Upper East Region of Ghana and in Togo using Envisat advanced synthetic aperture radar satellite images. The model is based on the Thornthwaite-Mather procedure and the assumption that with increasing precipitation, the contributing watershed area increases exponentially. The model parameters were estimated using the 2005 data and were validated using 2006 data. Although the total rainfall amounts were comparable in these 2 years, the rainfall and reservoir filling patterns were quite different. The model results indicate that the overall impact of the reservoirs largely depends on the ratios of reservoir to watershed areas. For this 2 year study, the reservoirs captured on average 34% of quick flow and 15% of overall runoff.

Suitability and Limitations of ENVISAT ASAR for Monitoring Small Reservoirs in a Semiarid Area

In semiarid regions, thousands of small reservoirs provide the rural population with water, but their storage volumes and hydrological impact are largely unknown. This paper analyzes the suitability of weather-independent radar satellite images for monitoring small reservoir surfaces. The surface areas of three reservoirs were extracted from 21 of 22 ENVISAT Advanced Synthetic Aperture Radar scenes, acquired bimonthly from June 2005 to August 2006. The reservoir surface areas were determined with a quasi-manual classification approach, as stringent classification rules often failed due to the spatial and temporal variability of the backscatter from the water. The land-water contrast is critical for the detection of water bodies. Additionally, wind has a significant impact on the classification results and affects the water surface and the backscattered radar signal (Bragg scattering) above a wind speed threshold of 2.6 mmiddots-1. The analysis of 15 months of wind speed data shows that, on 96% of the days, wind speeds were below the Bragg scattering criterion at the time of night time acquisitions, as opposed to 50% during the morning acquisition time. Night time acquisitions are strongly advisable over day time acquisitions due to lower wind interference. Over the year, radar images are most affected by wind during the onset of the rainy season (May and June). We conclude that radar and optical systems are complimentary. Radar is suitable during the rainy season but is affected by wind and lack of vegetation context during the dry season.

Hydrological parameterization through remote sensing in Volta Basin, West Africa

Abstract Ground‐based hydrological data collection tends to be difficult and costly, especially in developing countries such as Ghana and Burkina Faso where the infrastructure for scientific monitoring is limited. Remote sensing has the potential to fill the gaps in observation networks. The GLOWA Volta Project (GVP) seeks to maximize the information to be gained from satellite imagery by combining remotely sensed data with strategically chosen ground observations. However, there is very limited information about the coupling of remotely sensed data with ground based data over the mixed savanna terrain of West Africa. This paper provides an overview of innovative techniques to measure hydrological parameters as actual evapotranspiration, rainfall, and surface runoff over mixed savanna terrain in a semi‐arid region in West Africa, and their potential use. Evapotranspiration – The Surface Energy Balance Algorithm for Land (SEBAL) was used to calculate sensible heat flux and evapotranspiration through the energy balance. The SEBAL parameterization is an iterative and feedback‐based numerical procedure that deduces the radiation, heat and evaporation fluxes. Along a 1,000 km gradient in the Volta Basin, three scintillometers were installed to measure sensible heat flux over distances comparable to NOAA‐AVHRR pixels, approximately two kilometers. The comparison of sensible heat flux measured from remotely sensed data and scintillometers provide accurate results. This will help to increase the reliability of SEBAL parameterization. Rainfall – Depending on the region within the Volta Basin, up to 90% of the precipitation in originates from squall‐lines. The Tropical Rainfall Measuring Mission (TRMM) imagery provides a valuable tool to monitor such squall lines. However, the TRMM signal should be validated for squall line rainfall. To increase the reliability of space‐based rainfall measurements, TRMM based rainfall rate estimates were calibrated with rainfall measurements from a dense network of rain gauges. Surface Runoff – Remote sensing has limited value in estimating surface runoff. The savanna of West Africa, however, is dotted with a large number of small reservoirs used to supply water for households, cattle, and small scale irrigation. Bathymetry of sixty reservoirs in Ghanas Upper‐East Region produced a very regular correlation between surface area, as observable by satellites, and volumes. By using all‐weather RADAR imagery and the measured surface/volume curves, surface runoff volumes can be monitored throughout the year. These indirect runoff measurements will help researchers to develop surface‐runoff models for the Volta Basin.