12.6: Modifying the surface energy balance

Some land management practices are specifically designed to alter the surface energy balance. For example, people often cover some or all of the soil surface with materials intended to influence the surface energy balance and water balance and suppress the growth of undesired plants, and these materials are referred to as mulches. Common materials used for mulches include crop residues, leaves, wood chips, gravel, and plastic films.Crop residues are one of the most frequently used mulching materials, especially in cropping systems with no tillage or conservation tillage.Crop residue mulch tends to decrease the soil heat flux, resulting in cooler soil temperatures in the spring and summer and warmer soil temperatures in the fall and winter[12]. These changes in the soil temperature can have positive or negative effects on crop growth, depending on the circumstances.

Gravel mulches can increase the albedo of the surface, reducing the net radiation and evaporation, and they can decrease heat transfer between the soil and the atmosphere,again leading to cooler soil temperatures in the summer and warmer soil temperatures in the winter [13].Plastic films with a variety of colors and physical properties have been used as mulches, and in most cases,they increase the daytime soil heat flux and soil temperature. The greatest warming occur sunder films that have the highest ability to transmit shortwave radiation and absorb long wave radiation[14]. Plastic films also decrease or prevent evaporation and infiltration at the soil surface.

In addition to mulching, other land management practices can significantly alter the surface energy balance. In grasslands, springtime burning of accumulated plant residues is a common practice that stimulates new growth and helps prevent woody plant encroachment. It also causes significant changes to the surface energy balance. Energy balance measurements comparing burned and unburned tall grass prairie in Kansas, USA, showed that the burned area had 43% lower albedo,greater net radiation, and warmer soil temperatures than the unburned area[15]. This led to greater green leaf area and 23% greater latent heat flux and evapotranspiration totals for the growing season in the burned versus unburned prairie.The diurnal patterns of the surface energy balance terms for a representative day shortly after the burn are shown in Fig. 12-10.

Figure 12-6. Diurnal patterns of net radiation (Rn, a), soil heat flux (G, b), latent heat flux (LE, c), and sensible heat flux (H,d) from burned and unburned sites in the Konza Prairie near Manhattan, Kansas. Reproduced from Bremer and Ham (1999)

Even in urban areas, the surface energy balance is affected by land management practices. Measurements in Utah, USA, showed that soil heat flux was greater under concrete and asphalt covered surfaces than under surfaces covered by lava rock or pine bark mulch [16].Thus, the lava rock and pine bark mulch prevented more of the incoming radiation from entering the soil than did concrete or asphalt.The pine bark mulch resulted in the warmest surface temperatures, coolest soil temperatures, and greatest outgoing long wave radiation.