The Evaporation Conundrum

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Potential evaporation rates in many places in the world decreased during the second half of the twentieth century. As with solar radiation measurements, a major client for these measurements is the agricultural community, where evaporation rates are used to determine irrigation scheduling and application rates. Measurement of evaporation is usually done with an evaporimeter of the evaporation pan type, for example, the US class-A pan and Russian GGI-3000 pan [60]. Specifications of pan size, deployment and exposure are given in the previous reference. Networks of pans have been established in many parts of the world.

Evaporation of water requires large quantities of energy. Therefore, one model of evaporation is the energy budget of the evaporating surface, that is,

where Rn is net radiation absorbed by the surface, l is the latent heat of vaporisation, E is the evaporative flux, C is convective heat transfer with the environment and G is surface heat flux and/or energy storage. For annual totals, heat flux and energy storage can usually be ignored and evaporation depends only on net radiation and convection.

Evaporation from a wet surface (i.e. potential evaporation) can also be viewed as a diffusion process where water vapour is transported from the surface to the surrounding air, that is,

where p and cp are air density and heat capacity, respectively, es(Ts) and ea are water vapour pressure in air for saturation at surface temperature (Ts) and ambient conditions, respectively, and - is the psychrometric constant. r is the resistance to vapour transport from the wet surface to the point of interest in the air where humidity is measured, which in turn can be separated into a bulk surface resistance (rs) and boundary-layer aerodynamic resistance (ra). This second description of evaporation emphasises that it is influenced not only by radiation, but also by aerodynamic parameters like air temperature, humidity and wind speed, as well as surface parameters like roughness. Viewing both the energy budget and diffusion models of evaporation together, it is clear that climate factors determine the partitioning of radiative energy absorbed by a surface between the energy dissipation processes, that is, evaporation and convection.

The two approaches [Eqns (6) and (7)] can be used to solve for evaporation from a wet surface with few assumptions, giving the Penman equation [61], that is,

where Ta is air temperature, A is the slope of the relationship between saturation vapour pressure and temperature, and - is a bulk psychrometric constant which depends on surface properties. The expression (es(Ta)-ea) is the air vapour pressure deficit (VPD), which is a function of temperature and humidity. Thus, evaporation from a wet surface can be partitioned between radiative and aerodynamic influences on evaporation, where the radiative term (the left hand part of the Penman equation) is dominated by solar radiation and the aerodynamic term (the right hand part) depends on air temperature, humidity and wind speed. When analysing changes in potential (pan) evaporation Eqn (8) can help to determine which climatic factor has caused the change.

Widespread reductions in pan evaporation during the second half of the twentieth century were first reported for the former Soviet Union and much of the northern hemisphere [62,63]. These reports were considered evidence of global warming, which was thought to be increasing regional evaporation but decreasing pan evaporation due to a feedback influence of increasing regional humidity on local (or pan) potential evaporation [64] (see below). However, Stanhill and Cohen [23] considered decreasing evaporation to be evidence for decreasing solar radiation and Cohen et al. [65] showed that in Israel's arid conditions the overwhelming influence on evaporation is solar radiation. A full analysis of environmental factors showed that decreasing solar radiation was decreasing potential evaporation rates. Qian et al. [50] found a striking correspondence between decreasing Eg# and pan evaporation in China.

Two Australian biologists, Roderick and Farquhar [66], analysed worldwide changes in temperature and humidity and their relationship to evaporation rates. If regional evaporation were increasing and causing local pan evaporation to decrease then VPD should be decreasing [see Eqn (8)]. However, there was no evidence that this was occurring worldwide. Daily minimum temperatures are closely related to the daily dew point temperature and air vapour pressure (ea), since excess humidity precipitates as dew when the air is coolest in the early morning. Saturation vapour pressure (es) increases exponentially with increasing temperature, so if average and minimum temperatures increase at the same rates, VPD will increase and this should increase evaporation rates. However, worldwide minimum temperatures are increasing much faster than average temperatures and Roderick and Farquhar reasoned that this might be stabilizing VPD, as observed in climate data from the US. This implied that the aerodynamic term in the Penman equation [Eqn (8)] was stable; and if evaporation was decreasing it would have to be caused by decreasing net radiation, which is dominated by solar radiation. Roderick and Farquhar continued to develop a rigorous estimate of the evaporative equivalent to solar radiation. For a first order analysis the evaporative equivalent of radiative energy is expressed by l, whose value is ^2.4 MJkg 1 and 1 kg of water will cover a surface area of 1 m2 to a depth of 1 mm. For the region of the FSU where both radiation and evaporation trends were available, solar radiation, which was in the range of 3000 4000 MJm 2a 1, had declined by ^9% or 315 MJ m 2 in three decades, which is equivalent to 131 mm of water. This is similar to the average reported evaporation reduction during that period, ~111 mm of water. Thus, the reported reductions in evaporation rates matched those for solar radiation, and the pan evaporation data set corroborated the reported dimming trends in Eg#. Roderick and Farquhar's analysis [66] convinced many scientists that dimming was real and was having a significant impact on earth's climate.

Evaporation at most sites in Australia has decreased significantly during the period on record, with no signs of recovery during the 'brightening' era [67]. The climate parameters that could be causing this were investigated by Roderick et al. [68] using a physical model similar to Eqn (8). They found that the primary cause for the reduction in evaporation in Australia was decreasing wind speed with some regional contributions from decreasing solar radiation.

The question as to whether changes in pan evaporation are similar or opposite to changes in regional evaporation involves the 'complementary' hypothesis [69], which hypothesises that when regional evaporation changes, air humidity changes in the same direction, and a feedback occurs which has an opposite effect on local evaporation. The hypothesis [70] considers the sum of regional and local (e.g. pan) evaporation to be equal to a constant value, making them 'complementary'. For example, in the Tibetian plateau, Eg# and pan evaporation decreased from 1966 to 2003 [71], yet regional evaporation increased [72].

Since global radiation influences both local and regional evaporation similarly, when global radiation changes the constant of the complementary equation may also change. Nevertheless, when significant changes in air temperature occur, especially if accompanied by changes in wind speed, which have also been noted for many sites, changes in pan evaporation cannot be taken as unambiguous evidence for dimming, brightening or warming [73].

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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