The Interactions of Groundwater Systems and Land - Essay Example

?Changes in Earth’s Albedo Measured by Satellite (Wielicki et al. 825) The earth’s albedo refers to the reflected portion of the total solar radiation incident on earth. This fraction of the incident solar radiation, which is reflected back into space, is a “fundamental parameter” of the earth’s energy balance (Wielicki et al. 825). The global annual albedo of the earth is estimated to be approximately 0.29. A change of about 0.01 in the earth’s albedo equals approximately 3.4 W m-2 change in the global energy balance. This change is equal in magnitude to the effects caused if the carbon dioxide in the atmosphere is doubled! The global albedo is found to change with changes in the cloud cover, amount of atmospheric aerosols, and forest, snow, & ice cover on the earth’s surface. The magnitude of impact on global albedo due to events caused on earth can be demonstrated by the following example – A volcanic eruption in Mount Pinatubo in June 1991 released aerosols in the stratosphere, which raised global albedo by almost 0.007 in a span of two years. A larger increase in global albedo with unknown causes was reported to occur between 2001 and 2003. In order to investigate whether any significant changes in global albedo occurred between 2001 and 2003, Wielicki et al. examined the observations made by global satellites that measured changes in the earth’s albedo (825). These observations included those made by CERES (Clouds and Earth’s Radiant Energy System) of NASA’s Terra spacecraft. The monthly anomalies in the data caused by seasonal changes were nullified and data was plotted. The data provided by CERES covers global observations for the complete solar spectrum ranging from a wavelength of 0.3 to 4 µm. The observations of the global data reveals a small decrease of about 0.006 in the global albedo corresponding to about 2 W m-2 decrease in the shortwave reflected flux. These results contradict with those obtained by Palle et al, who demonstrated a large increase of about 0.017 in the global albedo corresponding to an increase of about 6 W m-2 in the shortwave reflected flux. Independent observations made by two individual CERES instruments were compared. It is believed that the 1.1 W m-2 decrease in the flux observed by one of the CERES instruments could be due to exposure to ultraviolet radiation during a hemispheric scan. When taken into consideration, this further reduces the anomaly to 0.9 W m-2. Wielicki et al. further explain the effect of change in albedo on earth’s climate (825). When changes in land surface, aerosols, and forest, snow and ice cover is the cause for change in albedo, then increasing albedo results cooling of the earth and decreasing albedo results in warming. These changes on earth’s surface significantly influence the amount of reflected solar radiation but have comparatively minor effects on the emitted thermal infrared radiation that results in cooling. Wielicki et al. contend that if observations made by Palle et al. were correct, then there would have been global cooling double of what had been observed in the Pinatubo eruption. However, such a global cooling was not observed. Furthermore, Wielicki et al. explain another possibility that the earth’s total ocean heat storage could witness a significant reduction. It is estimated that between 2000 to 2002, the ocean heat storage has experienced an increase of 0.7 W m-2. In order to account for the global changes in reflected solar flux, the flux in ocean heat storage was scaled to global surface area from an ocean-only area utilized by Willis et al (Wielicki et al. 825). According to Wielicki et al, if changes in global albedo were occurring, then there would be a decrease of 0.7 ± 0.8 W m-2 in the reflected flux. This is found to be consistent with the observations made by CERES. Until now, only the effects of change in albedo have been discussed. Cloud changes may also be affecting both the earth’s albedo and its thermal infrared cooling, and may also be a cause for albedo changes that do not affect the earth’s surface temperature and heat storage in oceans. Forests and Climate Change: Forcings, Feedbacks, and the Climate Benefits of Forests (Bonan 1444-1449) Forests cover about 30% of the earth’s land, equaling to about 42 million square kilometers of boreal, tropical and temperate lands. They serve a large number of ecological, social and economic purposes for the earth and humankind. They significantly influence the earth’s climate by moderating energy, carbon, water and other chemical exchanges with the atmosphere. Bonan thus contends that the world’s forests “influence climate through physical, chemical, and biological processes that affect planetary energetics, the hydrologic cycle, and atmospheric composition” (1444). An important fact to consider is that the surface albedo of forests is low and so, they can mask snow’s high albedo, resulting in planetary warming because of increase in heating of land through solar radiation. They also sustain the earth’s hydrologic cycle by evapotranspiration that results in climatic cooling through “feedbacks with clouds and precipitation” (Bonan 1444). In forests, the ratio of energy to evapotranspiration is generally low as compared to croplands. Furthermore, the ratio is higher in deciduous broad leaf forests than that in conifer forests. Forests, and their afforestation & deforestation affect the climate in many ways. These changes in climate may affect the ecosystem adversely. Therefore, Bonan suggests that forests can be “managed” in order to “mitigate climate change” (1444). Tropical Forests – Simulations in climatic models show that the rates of evapotranspiration maintained by tropical forests are high and that they decrease the temperature of the surface air and also cause an increase in precipitation. They contain about 25% of the total carbon present in the earth’s terrestrial biosphere. They are found to act as carbon sinks. They are thus important for both evaporative cooling and carbon sequestration. In spite of their lower albedos, their deforestation is found to result in warming due to reduction in evaporative cooling. Droughts may make them more susceptible to burning. Bonan argues that the “future of tropical forests is at risk in a warmer, more populous 21st-century world”, as warmer and drier climates are dangerous for their survival, thereby exacerbating global warming (1445). Global warming may cause a decrease in their evaporative cooling and result in an increased release of carbon dioxide that would cause forest dieback. Boreal Forests – Simulations in climatic models indicate that boreal forests have the “greatest biogeophysical effect of all biomes” on the global annual mean temperature (Bonan 1445). Boreal forests store large amounts of carbon. Their high albedo may offset the forcings that result from carbon emissions; therefore, deforestation of these forests would cool the climate. Boreal forests are susceptible to global warming. Temperate Forests – A large part of the temperate forests have been deforested for agriculture. The albedo of croplands is higher than that of forests and it is seen that compared to crops, trees are more likely to cause warmer surface air temperature. Temperate forests are found to result in a warmer climate because of their lower albedo. They have an uncertain net climate forcing. Deforestation of temperate forests may result in higher albedo, thereby offsetting carbon emissions, yet its net effect on climate is negligible. However, the reduction in evapotranspiration due to loss of forest cover may result in an increase in biogeochemical warming. Climate simulation models have also shown that the carbon cycle significantly influences climate. There appears to be a positive feedback between climatic warming and the carbon cycle. It is also found that biophysical cooling is attenuated by carbon emissions that result from land use. It can be concluded that forests, through their albedo, carbon cycle, evapotranspiration and other processes, can moderate climate change that results from emission of greenhouse gases. Flow and Storage in Groundwater Systems (William et al. 1985-1990) Groundwater is an important source of fresh water and a large proportion of people around the world rely on it as a primary drinking water source. It is also an important part of the earth’s hydrologic cycle, serving an important role in sustaining wetlands, lakes and streams. Groundwater is incorrectly viewed as a static reservoir of water and many are not familiar with its “dynamic nature” (William et al. 1985). However, in reality, groundwater flow systems are dynamic and show numerous interactions with land and surface water. A groundwater system includes subsurface water, the flow boundaries, the geologic media, which contains the water, the sources of water such as recharge and the sinks of the water, such as springs and wells. The time taken by water from the recharge to discharge area may range from a day to a million years. The hydraulic response time, i.e. the time required for the water levels to reach equilibrium after disturbance or hydraulic perturbation is estimated using the equation T*= SsLc 2 / K, where hydraulic response time is given by T*, specific storage is given by Ss­, basin length is given by Lc and hydraulic conductivity is given by K. The time of travel of water through a groundwater system depends on the “spatial and temporal gradients of hydraulic head, hydraulic conductivity, and porosity of the system” (William et al. 1985). Furthermore, the travel time varies from the hydraulic response time. The traveling time of water in a groundwater system is important for the determination of the movement of contaminants in it. It is seen that large capacity wells cause hydraulic which speed up the movement of the contaminants into the wells. Water initially withdrawn from a groundwater system comes from its reserve or storage, and as time passes, the effects of the water withdrawal propagate throughout the system. Gradually, the effect reaches a boundary where there is either increased recharge or decreased discharge. In the recent years, groundwater systems have been studied using computer models that predict flow. Tracer techniques have also been applied for the determination of residence time, recharge and discharge amounts and their timing. In the study of groundwater resources, recharge plays a critical role. However, recharge is difficult to quantify, and can be diffuse or localized. Most groundwater systems receive both types of recharge. The interactions of groundwater with surface water depends on the water bodies’ positions relative to the flow system, apart from their bed characteristics, the climate, and the underlying materials. The flow paths are affected by the geologic framework, and the sediment type at the groundwater and land interface affects the “spatial variability” of the discharge, in turn influencing biota distribution at the interface (William et al. 1988). The distribution of biota also depends on thermal effects, such as the temperature difference between groundwater and surface water. In presence of salt and freshwater, dynamic interactions can occur at the discharge boundary as well as in the ground, depending on the heterogeneity and homogeneity of the porous media. Layered mixing zones of water occur in case of heterogeneous porous media such as those found in coastal plain areas. The interactions of groundwater systems and land have also been extensively studied. Removal of groundwater from storage results in a lowering of the hydraulic heads. This process results in the transferring of a fraction of the mechanical support for the subsurface water and sediments from pore fluid pressure to the aquifer system’s granular skeleton. Removal of more and more water leads to excess pressure on the granular skeleton which is then compressed irreversibly, resulting in compaction of the interbedded aquitards. This can cause serious damage to structures, and may also result in problems in the designing and operations of drainage systems, flood protection etc. A proper understanding of the dynamic properties of groundwater requires further and more elaborate data collection, apart from better methodologies and modeling tools. Works Cited Alley, William, Richard Healy, James LaBaugh, and Thomas E. Reilly. “Flow and Storage in Groundwater Systems.” Science 296 (2002): 1985-1990. Bonan, Gordon. “Forests and Climate Change: Forcings, Feedbacks, and the Climate Benefits of Forests.” Science 320 (2008): 1444-1449. Wielicki, Bruce, Takmeng Wong, Norman Loeb, Patrick Minnis, Kory Priestley, and Robert Kandel. “Changes in Earth’s Albedo Measured by Satellite.” Science 308 (2005): 825. Read More