Satellites measure heat flux instantaneously as they pass over an area. Such instantaneous measurements (no more than the blink of an eye) poorly represent the daily average if variability in heat flux is high, as is the case in much of Africa. By combining Distributed Temperature Sensing and drone-based thermal measurements, we are able to get a detailed picture of variability in temperature and energy fluxes at the ground surface.

The Surface Energy Budget

Solar radiation reaching the Earth’s surface is generating fluxes of energy that play an important role in determining our weather and climate. The absorbed energy at the surface can be either emitted back to the atmosphere or transferred into the soil. Knowledge of the distribution of these fluxes, also referred to as the Surface Energy Budget is important because it forms the basis for many climate and weather models as well as predictions of crop yield.

Satellite sensors have been deployed to measure surface heat flux and derive estimates of evaporation. However under the often cloudy sub-Saharan African conditions, those satellite-based methods are not reliable and often fail. Another  important source of uncertainty in application of satellite-based methods in sub-Saharan Africa is that the ground heat flux is generally assumed to be negligible at a daily scale. In Sub-Saharan Africa, the ground heat flux can constitute up to 40% of the instantaneous energy balance and therefore it is very important to have in-situ measurements over the day and at various locations, to improve the satellite-based estimates of surface heat fluxes.

TU Delft, Future Water and KNUST measured ground temperature in time and space over a large area near Tamale in Ghana with the use of Distributed Temperature Sensing (DTS). DTS allows the instantaneous measurement of temperature along an optical fibre cable: every second, every meter, for kilometres of cable. This is possible because of a laser pulse that is emitted into the fibre-optic cable and is partly scattered back all along the cable. Part of the backscattered signal is temperature dependent, so temperature along the cable can be obtained for thousands of points along the cable (up to 8000 points per kilometre, to be exact).

In addition to the DTS measurements, FutureWater and HiView deployed Unmanned Aerial Vehicles (UAVs) to measure the spatial variability of surface temperature of the soil and vegetation. Using a thermal and near-infrared (NIR) camera, ultra-high-resolution images of ground temperature variations were made. Images of multiple flights during the day show how the surface heats up and cools down as the sun moves over the field.