The energy balance of the earth' surface : a practical approach

This study is devoted to the energy balance of the earth's surface with a special emphasis on practical applications. A simple picture of the energy exchange processes that take place at the ground is the following. Per unit time and area an amount of radiant energy is supplied to the surface. This radiation originates partly from the sun, but an~ other fraction is coming from the atmosphere (= infra-red radiation emitted by clouds, water vapour and CO _{<font size="-1">2</font>} ). From these gain terms the following losses must be subtracted: (a) the reflected solar radiation and (b) the infra-red radiation emitted by the surface itself. The final result is that a net amount of radiant energy is received by the surface, simply denoted as net radiation. At the ground net radiation is used to heat the ground (soil heat flux), to evaporate liquid water (evaporation), and to heat the atmosphere (sensible heat flux). In this simple picture we have neglected minor terms such as the energy used by the plants for their photosynthesis.Due to the high value of the latent heat of vaporization, the energy needed for evaporation is often an important term in the energy balance. In addition the energy balance of the earth's surface is linked with the water budget of both the atmosphere and the earth's surface, through the evaporation at the ground.Several practical questions in agriculture, hydrology and meteorology require information m the energy balance of the surface. It is the purpose of this study to find solutions for some of these problems.In hydrology one is mainly concerned in evaporation averaged over 1 day or more on a regional scale. Generally, this refers to land surfaces, but the evaporation of inland lakes or reservoirs is also of interest. In this context we also mention the problem of thermal pollution of open water bodies by industry or power plants. For this the so-called natural water temperature must be known, which is the temperature of the water in the hypothetical case that there is no artificial heating. It appears that this temperature depends mainly m the energy balance at the surface. In Chapter VI a model dealing with this problem is discussed.In agriculture one is interested also in evaporation. Now time intervals ranging from half an hour to several days are of interest.The relation between evaporation an the one side and plant diseases and pest control an the other can be mentioned.Furthermore, the yield of several agricultural crops is the greatest when the evapotranspiration is potential (= a maximum under the given weather conditions). When the crop transpires less than the potential rate, because the soil is too dry, the yield can be augmented by artificial precipitation. For applications such as these cheap and simple techniques are required for measuring the actual and potential evaporation. This applies also to agricultural research projects, e.g. to determine yield-water use relationships.In Chapter II simple measurement techniques are considered.Recent developments in meteorology have led to an increase of the interest in the energy balance of the earth's surface, especially in the input of heat and humidity at ground level into the atmosphere. Examples are models for the atmospheric boundary layer and related models for short range weather forecasts (12-18 h ahead). These models require simple parameterizations of the surface fluxes. This applies also to weather forecast models on a medium time range (3-10 days ahead).Since the height of the boundary layer is related to the heat input at the ground information an the surface energy balance is needed also for air pollution problems.In Chapter III a simple parameterization for evaporation and sensible heat flux is described that can be used for these type of problems.Usually, the only available data are standard weather observations. For that reason, many of the practical questions, mentioned above, can be formulated as: "How can the surface energy balance be estimated from standard weather data only'?" In Chapters III and VI possible answers to that question are discussed.Chapter II is devoted to simple measuring techniques that, in principle, can be used on an operational base. These methods will be compared with the so-called energy-balance method, using Bowen's ratio.In Chapter III two models for evaporation and sensible heat flux during daytime are compared. Both require standard weather data as input and an indication of the surface wetness. The first model needs more data, but contains more physics. The second is less complete, but requires less input data.Chapter IV has a mainly theoretical character. A model is presented that couples the evolution of the atmospheric boundary layer to the surface energy balance. It describes the course of the height, temperature and humidity of the boundary layer, together with the surface fluxes, when the initial profiles of temperature and humidity the radiative forcing and the surface wetness are known. It is restricted to convective conditions. Model output will be compared with observations.In Chapter V an empirical evaporation model for open water is considered. Comparisons with observations of evaporation of the former Lake Flevo will be made; the annual and the diurnal cycle will be considered.In Chapter VI a model for the (natural) temperature and energy balance of inland lakes and water reservoirs is discussed that requires standard weather data only. A comparison between the calculated and measured water temperature will be given. This concerns two adjacent water reservoirs, which have about the same size, but which differ in depth (5 and 15 m). This is of importance, since the water temperature also depends on water depth.At some places we made new modifications, but most of the theoretical concepts applied in this study are adopted from literature. This is inherent in our practical approach. Some of the theories used have been available for many years. But, e.g. because no suitable instruments were available, they were not usefull for practical applications. Recent developments in the field of instrumentation and data handling have changed the situation-to our advantage. A good example is the temperature fluctuation method for measuring the sensible heat flux (discussed in 11.4). The theoretical basis for this approach was given by Prandtl already in 1932. But for an experimental verification we had to wait until the sixties and early seventies. In that period instruments were developed to measure turbulent surface fluxes and fast temperature fluctuations, while also the data handling techniques were improved significantly. Finally) the method wouldn't be operationally until quite recently.For the verification of the parameterizations, measuring techniques and models treated in this study, we used data collected at the 200 m mast at Cabauw, and at the nearby micrometeorological field, of the Royal Netherlands Meteorological Institute