How hot is the soil?

The Surface Energy Budget

While the sun is beating down on the Earth’s surface, it is generating fluxes of energy that play an important role in determining our weather and climate. In a way, it functions as a gigantic solar panel that absorbs solar radiation. The absorbed energy can be either emitted back to the atmosphere or transferred into the soil. Or, when solar radiation hits water or vegetated surfaces, solar energy can be used for evaporation, sending water vapor back into the atmosphere. Knowledge of the distribution of these fluxes, also referred to as the Surface Energy Budget (see figure) is important because it forms the basis for many climate and weather models as well as predictions of crop yield.

Surface Energy Budget (image credit: NASA)

Satellites blinking at the surface

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 the 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. This is generally true if the incoming heat flux during the day is of similar magnitude as the outgoing heat flux during the night. However, satellites measure heat flux instantaneously, as they pass over an area. , and such instantaneous measurements (no more than the blink of an eye) poorly represent the daily average if variability in heat flux is high. 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.

Optical fibers, a thousand thermometers in the soil

In TWIGA, we measured ground temperature in time and space over a large area with the use of Distributed Temperature Sensing (DTS). DTS allows the instantaneous measurement of temperature along an optical fiber cable: every second, every meter, for kilometers of cable. This is possible because of a laser pulse that is emitted into the fiber-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 kilometer, to be exact).

Installation of DTS cables in the soil at field site KNUST

UAVs watch surface heating from the sky

In addition to the DTS measurements, FutureWater and HiView deployed Unmanned Aerial Vehicles (UAVs) to measure the spatial variability of the 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.

Temperature images of the soil field site at Nyankpala taken by UAV four times of the day (upper row) and histograms of the temperature distribution across the field (lower row)

With the DTS and UAV measurements, we are able to get a detailed picture of variability in temperature and energy fluxes that are important to assess the quality of satellite-based energy flux estimations. Additionally, the temperature information can be used to derive information on soil moisture that in turn is crucial for crop planning and yield. Two Ph.D. students have started at KNUST to look deeper into what we can learn from the soil temperatures.

Written by Marie-Claire ten Veldhuis