Heat Environmental Characteristic of Storm Water Reservoir with Various Aquatic Plants
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HEAT ENVIRONMENTAL CHARACTERISTIC OF STORM WATER RESERVOIR WITH VARIOUS AQUATIC PLANTS
YASUSHI MABUCHI
Graduate School of Eng., Infrastructure Systems Engineering Dept., Kochi University of Technology Tosa Yamada-cho, Kochi Prefecture 782-8502, JAPAN
TOMOKO NAKATA
Graduate School of Eng., Infrastructure Systems Engineering Dept., Kochi University of Technology Tosa Yamada-cho, Kochi Prefecture 782-8502, JAPAN
MASAHIRO MURAKAMI
Infrastructure Systems Engineering Dept., Kochi University of Technology Tosa Yamada-cho, Kochi Prefecture 782-8502, JAPAN
The Ishizuchi reservoir in Kochi prefecture comprises a rich natural ecosystem including variety of aquatic plants. Field measurements have been carried out in September 2003 to evaluate the cool island effect of typical aquatic plants in the reservoir during summer season. The effect of reduction in the primary energy and carbon flux are calculated using the latent heat coefficient of several kinds of aquatic plants. These effects are then used to evaluate the environmental economic cost from the viewpoint of heat environment of the reservoir. The results shows that the rate of latent heat flux in the net radiation in the case of reservoir with free-floating and emergent type of aquatic plants is 2 to 3 times higher than the case of reservoir without aquatic plants. This evapo-transpiration from the aquatic plants is about a half of the evapo-transpiration of from the grassland. The environmental economic benefit in the summer season is evaluated to be 10.53 million yen per day by reducing the primary energy and the carbon flux. This result is 48% higher than the case of reservoir without aquatic plants, corresponding to 0.15% of the GDP in Kochi prefecture
INTRODUCTION
Forests and waterbodies are considered as major factors of mitigating the urban climate in large cities. Many case studies on the heat environment estimated the cool island effect (Honjo,2000,[1]; Harazono et al,1992,[2]). In general, reservoir contains aquatic plants. Therefore, the photosynthesis activity occurs not only on land, but also in the reservoir. The evaporation process in reservoirs depends not only on the evaporation from water surface but also on the transpiration from aquatic plants. The cool island effect on the reservoirs with aquatic plants has long been discussed to estimate the evaporation from the water surface (Nayakama et al, 1993, [3]).
The objective of this study is to estimate the cool island effect of typical aquatic plants in the reservoir during the summer season. A series of field measurements have been carried out to estimate the heat environment of typical aquatic plants in the Ishizuchi reservoir of Kochi prefecture in September 2003. The reservoir has a rich natural ecosystem including variety of aquatic plants. The environmental economic benefit is evaluated to measure the effects of reduction in the primary energy and the primary energy reduction effect and the carbon flux by taking into account the latent heat of several kinds of aquatic plants.
OUTLINE OF STUDY AREA
Ishizuchi reservoir was constructed in 1990 to store the excess urban storm drainage in the Tochi newtown in Kochi prefecture (Fig.1). The reservoir has a rich ecosystem including a variety of aquatic plants such as lotus and water hyacinth, and aquatic faunas including turtle, fishes and waterfowl. Moreover, the reservoir has a beautiful landscape, and the waterfront is surrounded by resorts and recreational facilities where the residents can enjoy walking, fishing and bird watching etc. The Local government is investigating a water purification method using aquatic plants to not only prevent further pollution but also restore the good quality of water environment (RIZA, 1999, [4]).
Figure 3. Typical aquatic plants in the reservoir
The surface of Ishizuchi reservoir is covered with (1) extensively lotus in the southern and the western part, (2) water chestnut in the east, and (3) water hyacinth on the shoreline in the north. About 80% of the reservoir’s surface is covered by aquatic plant (Figs.2, 3).
FIELD MEASUREMENT IN ISHIZUCHI RESERVOIR
Measured parameters
Field measurements were carried out at Ishizuchi reservoir in 2-3 September, 2003 using a scintillation measurement system (Sintec Co., Ltd. Sys-40) and integrated meteorological observation system. The field measurement aims to evaluate the cool island effect on the aquatic plants, to compare the effect of evapo-transpiration on the aquatic plants and direct evaporation from the water surface.
The following parameters such as air temperature, humidity, global radiation, long wave radiation (upward and downward) and underground heat flux were measured using an integrated meteorological observation device. At the same time, the amount of heat flux in water hyacinth was measured continuously for 24 hours using scintillation measurement system. Moreover, to evaluate the thermal characteristics of each aquatic plant which cover the reservoir surface, an infrared thermal imaging camera is placed at the top of a hill nearby the reservoir.
The results of field measurement
A temperature measurement of the reservoir’s surface which is covered with some kinds of aquatic plants was carried out to distinguish the difference of surface temperature in the reservoir. Fig.4 shows daily changes in surface temperature on the lake surface which is covered with different type of aquatic plants.
Figure 4. Daily changes in surface temperature on the lake surface which is covered with different type of aquatic plants
The water surface temperature is about 4oC higher than the grass and aquatic plant which is covered in the reservoir during the daytime. The water surface temperature is about 1.5oC lower than green tract of land use in the night. It is conceivable that the thermal conductivity of water body is low as 1.0 to store the excess heat flux in the daytime. The surface temperatures among the different three types of aquatic plants were measured to be 34.3oC on the water hyacinth, 33.3oC on the lotus, and 33.3oC on the water chestnut. The maximum difference among the three amount to 1.5oC.
HEAT ENVIRONMENTAL CHARACTERISTICS OF AQUATIC PLANTS
This research is carried out to evaluate the heat environmental characteristics of each aquatic plant using the results of field measurement. The cool island effect of surface cover is estimated to use the water hyacinth, lotus, water chestnut and water surface in the reservoir. The pedestrian along the reservoir, which is covered with green grass, is also used to compare the results.
The quantification method of heat environment
The quantification method of the heat environment of lakes and marshes with various aquatic plants basically applies the heat balance equation, and the calculation of each item uses the bulk model. The bulk model is an estimation method for various flux from the surface temperature, the temperature and humidity of optional altitude, and the bulk coefficient under the assumption that the change of heat capacity of the atmosphere and the ground surface is equal to the air temperature difference. The equations of the sensible and latent heat flux by the bulk model are shown in the following equation, cf. (1) – (5). The advection parameter was omitted because it was smaller than other heat balance parameters.
(1)
(2)
(3)
(4)
(5)
Where Cpρ is the volumetric heat capacity of the atmosphere (1hPa,20:1.21×103JK-1m-3). CH, CE is sensible heat and latent heat bulk coefficient. Ts is surface temperature (oC). Qs is saturated specific humidity for Ts (kgkg-1). E is the amount of evaporation (kgm-2s-1). U is wind speed (ms-1). T is air temperature (oC). q is specific humidity(kgkg-1). z0,zt,zq is roughness for wind speed, air temperature and specific humidity (m). d is zero-plane displacement(m). k is Karman constant (0.4).
As various parameters of the bulk type are set, the same measurement procedures as in Kondo et al. (1994) is applied in this study (Table 1) (Kondo, 2000, [5]).
Table 1. Bulk equation coefficients
Heat environmental characteristics of aquatic plants
The latent heat and the sensible heat flux in the reservoir are very small to compare with the amount of net radiation. The G/Rn was 0.8 or more during the whole period of field measurement. Most of the net radiation is accumulated in the reservoir water. In the case of reservoirs with aquatic plants, the rate of latent heat flux in the net radiation, is increased 2 to 3 times to compare with the case of reservoir with free-floating and emergent type of aquatic plants. This result shows that the effect of evapo-transpiration from aquatic plants is about 1/2 of evapo-transpiration from the green grassland. The peak value of the latent heat flux was estimated to be 300W/m2 for water hyacinth and lotus, and 200W/m2 for water chestnut.
Figure 5. Heat balance using bulk model
The peak value of sensible heat of water chestnut is assumed to be 0W/m2. The sensible heat of water hyacinth and lotus were estimated to be 50W/m2. It is conceivable that water chestnut is distributed in the reservoir surface, owing to its small leaf area in the aquatic plant. While the leaves of water hyacinth and lotus are directly heated by sunlight to yield the sensible heat. This shows contribution to the increase of ambient temperature. The latent heat and the sensible heat flux in the vegetation cover with aquatic plants increase to compare with the water surface without aquatic plants. The heat environmental characteristic of water surface with aquatic plants in the reservoir is between the water surface without aquatic plant and green grassland on the shore (Fig.5).
ENVIRONMENTAL ECONOMIC ANALYSIS OF VARIOUS AQUATIC PLANTS
The environmental economic benefit, from the viewpoint of heat environment of the reservoir, is evaluated from the primary energy reduction effect and the carbon flux by the latent heat of different types of aquatic plants.
Estimation of primary energy reduction effect
In general, the vegetation area and waterbodies such as lake surface area are prominent for the latent heat flux. When water evaporates, the vaporization heat decreases the peripheral temperature (this effect of evapo-transpiration from plants and water surface is equal to the natural air conditioner). The environmental economy of cool island effect from plants and water surface can be evaluated by substituting the energy used for latent heat by the equivalent amount of the oil energy used by the air conditioner. The amount of primary energy reduction is estimated from the amount of evapo-transpiration of the plants and the water surface [6].
Eo = Wp * Wh / Oe(6)
Ee = Wp * Wh / Eh(7)
Ec = Ee * C(8)
Where, Eo is oil equivalence (t). Wp is evapo-transpiration from plants and water surface(g). Wh is heat of vaporization of water (583cal/g constant). Oe is the amount of energy obtained when oil burns completely (107kcal constant). Ee is the amount of oil energy corresponding to evapo-transpiration from plants (kWh). Eh is the heat capacity obtained by using electric power (1kWh) in output base (860kcal constant). Ec is the primary energy reduction effect (yen/day). C is electricity bill (25yen/kWh constant).
Table 2 shows the environmental economic benefit of cool island effect for each type of aquatic plant in Sep. 2, 2003. Evapo-transpiration on each land cover was calculated from the latent heat flux that had been measured from field measurement, so this environmental economic analysis is an evaluation at the time of field measurement. The results indicate that the average evapo-transpiration per unit area in Ishizuchi reservoir was estimated to be 2.41mm/day, and the evapo-transpiration energy of whole of Ishizuchi reservoir was calculated to be 387,000 kWh. The energy of conversion from electricity to oil was estimated to be 33t/day. This quantity of electric power corresponds to 2.5% of the amount of power consumption per day in Kochi Prefecture (15.21 million kWh). The effect of evapo-transpiration in the reservoir is evaluated to be 9.67 million yen (91.1 thousand US$) per day to reduce the primary energy.
Table 2. Environmental economic cost of cool island effect for each type of aquatic plant on Sep. 2, 2003
Estimation of carbon flux
Recently, the international trading of the carbon dioxide is discussed between the developed countries and developing countries. The carbon tax is a kind of the environment tax, and methodology for stimulating approach that reduces the environmental destruction. Concretely, it is an economical policy instrument that controls the excess amount of carbon dioxide (CO2) by adding the tax on the crude oil, coal and natural gas. In Japan, carbon tax will be introduced in 2005 (Ministry of Environment, 2001[7]).
Cl = Eo * Co(9)
Et = Cl * Ta(10)
Where, Cl is the amount of carbon exhaust on each land cover(t/day). Co is the amount of carbon consumption per 1 litter crude oil (0.86t constant). Et is a trading carbon tax (yen/day). Ta is the carbon tax per 1 litter of carbon (30,000 yen, 283 US$ constant).
The cool island effect of Ishizuchi reservoir is will replace the excess carbon dioxide corresponding to 28.62t/day during summer which is equivalent to 0.86 million yen (8.1 thousand US$) per day of reducing primary energy. The amount of the primary energy reduction and the carbon flux is estimated to be 0.15% of the GDP in Kochi prefecture (Table 3).
Table 3. Carbon flux on cool island effect after introducing carbon tax
Table 4 shows the environmental economic evaluation of aquatic plants in the Ishizuchi reservoir. Heat environmental economy of current Ishizuchi reservoir with aquatic plants is estimated to be 3.1 million yen (29.2 thousand US$) to reduce the primary energy, and about 178,000 yen (1.6 thousand US$) to reduce the carbon tax in the case of reservoir without aquatic plants. The aquatic plants in the Ishizuchi reservoir demonstrate the effect of mitigating the urban climate in summer.
Table 4. Environmental economic evaluation of aquatic plants in the Ishizuchi reservoir
CONCLUSION AND REMARKS
The result of this summarized as follows;
(1) The latent heat and the sensible heat flux are very small to compare with the amount of net radiation in the reservoir water aquatic plants. The G/Rn was 0.8 or more during the whole period of field measurement.
(2) The rate of latent heat flux in the net radiation in the case of reservoirs with aquatic plants, is increased 2 to 3 times to compare the case of reservoir with free-floating and emergent type of aquatic plants. It is also estimated that the evapo-transpiration from aquatic plants is about 1/2 of evapo-transpiration from the green grassland.
(3) The environmental economic benefit in the peak summer season is estimated to be 10.53 million yen per day to reduce ether the primary energy electricity or carbon dioxide. The case with aquatic plants is 48% higher than the case of reservoir without aquatic plants in the reservoir, corresponding to 0.15% of the GDP in Kochi prefecture
REFERENCE
[1] T. Honjo: ”Themal Environment of Urban Green Areas” Jounal of Agricultural Meteorology, Vol.56 (3), pp227-233, (2000).
[2] K. Narita, A. Uemura, I. Misaka: “Observations on the thermal effects of river water on urban climate” Japan Architecture Planning Environ. Eng., No.545, 71-78, (2001)
[3] K. Nakayama, T. Urano, H. Kon, N. Matsuoka: “The effects of the generation of water blooms on the Heat Balance in water body” Journal of Agricultural Meteorology, Vol.48 (4), pp359-363, (1993)
[4] RIZA: “Stoneworts : valuable for water management” pp4-26, (1999)
[5] J. Kondo: “Meteorology of water environment”, pp101, (2000)
[6] “Energy unit equivalence”, energy_ equiv.html
[7] Ministry of Environment: “Central Environment Council interim report”, pp151, (2001)
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