Role of Plants in the Water Cycle Linkage among Evapotranspiration and Energy Fluxes Dissipation

Vegetation and water presence in the ecosystem sub stantially determine distribution of incoming solar radiation. In a landscape with sufficient supply of water, the prevailing part of the incoming radiation can be dissipated through vapor of water (latent heat), whereas in the dry landscape most of the incoming radiation must turn into sensible heat (see Figure 5). ET reflected as latent heat represents the most dynamic part of the whole system. Due to the fact that transpiration is a controllable process, it is the vegetation that largely controls the ET rates. If water supply in the ecosystem is sufficient, the transpiration rate can be very high; on the contrary, under dry conditions, plants close their stomata and save as much water as possible, simultaneously limiting evaporation by shielding the ground. In the temperate zone, on sunny days of the vegetation season, the ground heat flux is commonly 5-10% on dry lands and up to 20% on wet lands. The energy accumulated in biomass through the processes of photosynthesis is minor. Only about 0.45% of the approximate annual global radiation of the temperate zone (1100 kWhm ) is used for biomass synthesis. The most important variable influencing the fate of solar energy in the ecosystems is the water availability (Figure 5).

Drainage and suppression of vegetation on large areas are associated with release of huge amounts of sen sible heat (several hundreds of watts per square meter on a sunny day) and formation of heat potentials. A decrease of ET of 31 from 1 m per day results in an increase of sensible heat of 2.1 kWh (7.5 MJ). Drainage

Water flow

Heat 60-70%

0-1000 W m-2 flow

Daily input of solar energy 6 kWh m-2

Water vapor ET 70-80%

Reflection 5-20%

Heat flux 5-10%

Heat flux 5-10%

Figure 5 Distribution of solar energy in a drained landscape and in a landscape well supplied with water.

Reflection 0-10%

Heat flux 5-15%

Sensible heat 5-10%

Lake, meadow, forest Drained land Landscape saturated with water

Figure 5 Distribution of solar energy in a drained landscape and in a landscape well supplied with water.

of 1000 ha (10 km2) accompanied with decrease of ET by 3 mm (3 l/m -day) results in a daily release of sensible heat of 2.1 x 107 kWh (2.1 x 104MWh or 21 GWh).

Growth of vegetation is associated with fixation of carbon dioxide, whereas vegetation decline and decom position of biomass are associated with its release. The average annual production of dry biomass is 0.5 kg m . On an area of 100 000 km2, 50 x 106 metric ton of biomass is produced. As dry biomass contains 40% of C, about 20 x 106 metric ton of C is fixed in this amount of bio mass, for which about 70 x 106 metric ton of carbon dioxide is captured from the air into the plant tissue. At the same time, if present, at least 50 x 109 metric ton of water is evaporated via ET during a vegetation season (500 l m 2), which represents air conditioning effect of 35 x 109MWh. During a sunny day, the ET of 3 mm from 100 000 km2 represents 3 x 10111 of evaporated water (air conditioning effect 2.1 x 108MWh).

Drainage Paradigm

The water transpired by plants is often considered a loss, especially in the drainage paradigm of the older hydrological and technological literature. Transpiration is sometimes even called an unavoidable evil, in the sense that water is sacrificed for the sake of enabling intake of CO2 for photosynthesis processes. This problematic attitude toward meaning of tran spiration has also been reflected in a controversy on the hydrological role of forests in the hydrological budget of small catchments. When comparing relatively small catch ments, less rainfall is converted to the runoff from afforested catchments than from meadows (grass covered) catchments or partly drained catchments. This has been proved many times in hydrological studies in small catchments. A 5 year comparative study of small catchments in the Sumava Mountains (Czech Republic) showed 40% higher water run off to the recipient from a drained pasture in comparison to afforested and wetland catchments (see Table 2). However, the runoff was unbalanced and the drained landscape became vulnerable to the weather variability.

The 'ameliorative' lowering of groundwater table, which is still in progress, has led to rapidly oscillating periods of drying and wetting combined with rapidly changing surface temperature of often extreme values. This evolution has contributed substantially to acidifica tion of the topsoils by oxidation of organic matter and destruction of soil colloids. Both organic matter and soil colloids are needed for matter and water retention. The oxidation processes result - most often in the summer when the water table decreases due to enhanced tempera ture and evaporation - in production of strong nitric and sulfuric acids. Subsequent weathering of inorganic soil minerals into easily dissolvable salts results in enor mous non point source eutrophication processes (see Figure 6). This happens mainly in autumn, under condi tions of rising groundwater tables and increased water

Table 2 An example of measured annual water budgets (precipitation and water discharge, in m3 ha 1 yr 1) in three small catchments (c. 250 ha each) in Sumava mountains, Czech Republic, in 5 years period (2000-2004). The decrease of the ratio of water precipitated to water discharged is caused mainly by evapotranspiration in the catchment

Table 2 An example of measured annual water budgets (precipitation and water discharge, in m3 ha 1 yr 1) in three small catchments (c. 250 ha each) in Sumava mountains, Czech Republic, in 5 years period (2000-2004). The decrease of the ratio of water precipitated to water discharged is caused mainly by evapotranspiration in the catchment

System

Mode

2000

2001

2002

2003

2004

Average

Remaining water (%)

Drained pasture

Precipitation

11 019

9339

12 851

8968

9350

10 305

Water discharge

10 934

7339

12 438

7751

8105

9313

10

Peat meadow

Precipitation

11 935

10 0e5

15107

9494

9934

11 307

Water discharge

e 558

5382

8394

6448

6747

e 706

41

Forest

Precipitation

11 935

10065

15107

9494

9934

11 307

Water discharge

4623

4778

6451

4862

5495

6242

54

Postglacial pioneer coenosis

Long water cycle

Long water cycle

Uneven water [ discharge

Climax coenosis, short circuited water cycle, accumulation of biomass

Even water discharge

Climax coenosis, short circuited water cycle, accumulation of biomass

Even water discharge

Irregular precipitation

Area of matter losses Unsaturated soil zone

Cultural coenosis with accelerated water and matter discharge

Decomposition of organic matter

Area of matter losses Unsaturated soil zone

Decomposition of organic matter

Figure 6 Development of water cycle and matter losses from the soil in temperate zone after the last glaciation period. Adapted from Ripl W, Pokorny J, Eiseltova M, and Ridgill S (1994) A holistic approach to the structure and function of wetlands, and their degradation. In: Eiseltova M (ed.) IWRB Publication 32: Restoration of Lake Ecosystems - A Holistic Approach, pp. 16-35. Slimbridge, UK: International Waterfowl & Wetlands Research Bureau.

outflow when the dissolvable salts are transported via groundwater to the rivers, lakes, and coastal areas, causing both leaching of nutrient from the ecosystems on the one hand and eutrophication of the surface water on the other. Loss of vegetation also substantially contributes to the soil vulnerability to meteorological fluctuations, playing its role in the soil conservation, creation, and amelioration. Also, hydraulic redistribution of water in the soil may be to a large extent dependent on vegetation.

Land surface is an important part of the climatic sys tem. Large scale deforestation, drainage, and vegetation removal leads to change of hydrological cycle at local as well as regional scales. Changes in surface energy budgets as a consequence of changed ecosystem feedbacks can have a profound influence on the Earth's climate. Changes in forest cover in the Amazon Basin have affected the flux of the moisture to the atmosphere, and hence regional rainfall. It was shown that from all of the Lesser Antilles, the widely forested Dominica received more rainfall that other deforested islands. Land use in tropical lowlands has serious impacts on ecosystems in adjacent mountains, changing apart from others the cloud base heights with its possible devastating consequences on cloud forest ecosystems. A decrease of precipitation and/ or unbalanced precipitation (short periods of heavy rains followed by long periods of drought) results in degradation of vegetation cover, followed by increasing erosion and desertification. Such processes have been in progress recently, for example in Australia, where large areas of the landscape are drained and agriculturally overexploited. Well documented examples of such consequences are the profoundly increased drought risks in the Mediterranean and in the the Sahel region as a consequence of landscape overexploitation.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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