4.1 Estimating radiation budgets using elevation
Many environmental processes are dependent on terrain and its effects on insolation (incoming sunlight). For example, in the northern hemisphere patches of snow will often last longer into summer on north-facing slopes and trees may grow more quickly on sunnier south-facing slopes. The rate of evapo-transpiration may be lower in shadowy valley bottoms. This object examines how Digital Elevation Models (DEMs) can be used to assess the effects of terrain on radiation budgets. Consideration of the numerous factors influencing the impact of insolation on a land surface helps to reveal the importance of using DEMs rather than two-dimensional GIS representations for practical management operations. A thought-provoking short article on how slope and aspect impact on insolation and thus on the flavour of a glass of wine is presented by Wooldridge and Beukes (2005).
Insolation is defined as ‘solar radiation received at the Earth’s surface
The standard GIS tools – slope, aspect, and analytical hillshading
Many of the standard GIS tools can help to understand how radiation budgets vary with terrain. Most GIS software can calculate slope (derived from the relative elevation of adjacent locations) and aspect (direction of slope), which will indicate whether slopes are north- or south-facing. Many GIS systems also calculate the amount of light falling on different grid squares within a DEM. This is sometimes known as analytical hillshading or relative illumination.
Within Idrisi, for example, the HILLSHADE tool in Spatial Analyst calculates relative illumination across a DEM. Typically, such calculations assume that the sun is at a fixed point. The sun’s height is specified as an azimuth (i.e., an angle to the horizon) and its relative position as a compass bearing. The output map often has arbitrary units. Illumination is not measured in KiloJoules per pixel or any other quantifiable units of measurement, but on a relative scale from one grid square to the next.
More sophisticated tools – the solar radiation toolset
More recently, more sophisticated methods of calculating radiation budgets from DEMs have emerged, such as the Solar Analyst software (Fu and Rich 1999). Since version 9.1 of ArcView, Solar Analyst has been incorporated into the Spatial Analyst part of the ArcToolBox as a set of solar radiation tools. These tools work by breaking up the total solar radiation at a given site into three components: direct radiation, diffuse radiation, and reflected radiation. These concepts and the operation of the solar radiation tools are explained in the slide sequence below.
The Solar Analyst software differs from analytical hillshading GIS tools for calculating illumination in several respects:
- Firstly, Solar Analyst takes into account the effects of the surrounding terrain. In other words, unlike standard GIS tools, Solar Analyst recognises the fact that a hill may cast a shadow onto the surrounding land and lower insolation levels there.
- Secondly, unlike standard GIS hillshading, it produces insolation estimates that are in quantified units, such as KiloJoules per metre square.
- Finally, the Solar Analyst software explicitly takes into account the sun’s movement across the sky at a given location and time of year, unlike the standard hillshading tools, which assume the sun’s position is constant.
- The Solar Analyst software requires more input information than an analytical hillshading calculation.
For these reasons, the Solar Analyst software provides a more accurate representation of insolation across a study area than standard GIS tools can. Does this mean we should always use tools such as Solar Analyst or the Spatial Analyst solar radiation tools in environmental management?
Deciding on the right tool for different management situations
Three different environmental management scenarios are described below. For which of these scenarios do you think it would be appropriate to use more specialised applications such as Solar Analyst or the solar radiation tools? Why?
Scenario 1: As an analyst working for an environmental consultancy, you are investigating possible sites for generating energy through solar panels. You have a DEM for your site and wish to plan the exact locations for your solar panels. Would you use the solar radiation tools for this task or not?
Scenario 2: You are a GIS analyst working for a National Park authority. You are producing a map for the park’s web site that will be displayed to the general public. The map will show current land use within the park and how this relates to terrain. You wish to create a map that shows land cover, but with shading to indicate areas of mountainous terrain within the park. You have DEM and land cover maps available to you. Would you use the Solar Analyst for this task or not?
Scenario 3: You are a GIS analyst working for a National Park authority. You have a point map that shows the location of a rare orchid within the park and believe that it may prefer sunnier sites within the park. You wish to investigate this idea using GIS. Would you use the solar radiation tools for this task or not?
References (Essential reading for this learning object indicated by *)
* Fu, P., and Rich, P. M. (1999) Design and Implementation of the Solar Analyst: an ArcView Extension for Modeling Solar Radiation at Landscape Scales. Proceedings of the ESRI User Conference 1999. professorpaul.com/publications/fu_rich_1999_esri.pdf
Whiteman, D. (2000) Mountain Meteorology: Fundamentals and Applications. Oxford University Press.
Brito, M., Gomes, N., Santos, T., and Tenedorio, J. (2012) Photovoltaic potential in a Lisbon suburb using LiDAR data. Solar Energy 86 (1), 283-288. http://www.sciencedirect.com/science/article/pii/S0038092X11003574 This article provides an example of Solar Analyst being used for planning siting of domestic renewable energy.