Water and Energy Balances within the Jornada Basin

Abstract

This chapter describes general characteristics and components of the energy and water balances in arid regions, with specific examples from the Jornada Basin. Various research efforts to characterize the energy and water balances and resultant carbon dioxide fluxes in the Jornada Basin are detailed. We provide a brief overview of how plant physiology interacts with energy and water balances in this region, and characterize general abiotic conditions and some physiological traits of plants in this arid region. The surface of a landscape may be considered as a layer with some amount of vegetation. More general descriptions divide the vegetation, like the soil, into layers, but the concern here is energy balance at the interface with the atmosphere. The net energy balance of the land surface is determined by inputs (radiant energy), outputs (reflection [i.e., albedo], emission of longwave radiation, convective heat transfer to the atmosphere [i.e., sensible heat flux], evapotranspiration of water [i.e., latent heat flux], and conduction of heat into soil), and changes in heat storage. The balance of these terms is adjusted as the surface temperature comes into steady state or nearly so. Increased solar input will drive surface temperatures higher until longwave emission and other losses come into a new balance. The net energy input, as inputs minus outputs, may be stated formally as an energy-balance equation . . . Rate of heat storage = S = Q+sw + Q+TIR − Q+TIR _ Q_E Q_H − Q_S, (8-1) . . . where the superscript + indicates an input, and − indicates an output or loss, and all terms are expressed as flux density in units of W/m2. Q+SW is the energy added to the surface layer by solar radiation from above. Q+TIR is the thermal infrared radiation emitted by gases in the atmosphere, principally water vapor and CO2, whereas Q_TIR is the thermal infrared radiation emitted from components of the Earth’s surface and lost back to the atmosphere. Q_E is the latent heat flux from the heat of vaporization of water vapors resulting from soil evaporation (E) and plant transpiration, generally measured as the composite evapotranspiration flux (ET).