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Poster Session 5
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Land Cover Change and its Effect on Runoff in the Doki River Catchment
Various evapotranspiration estimates
Evapotranspiration is estimated by water budget method, direct observation with a pan
evaporator, and the modified Penman method from meteorological data. A pan evaporator
indicates evaporation from water body and means potential evaporation, the maximum
evaporation, and independent values from land use depending on meteorological conditions.
The Penman method overestimates evapotranspiration each land cover. These three methods
were compared on average separating the data in 1986 shown in Table 4. All the values
decreased commonly beyond 1986. The rates of their decrease were almost equal, ranging 85
to 90 %. Therefore, the change of evapotranspiration was brought by meteorological factors;
the contribution of land cover should be small.
Table 4. Evapotranspiration change using water balance, pan evaporation and Penman’s
method. (Unit: mm)
| Period |
Water Balance |
Pan Evaporation |
Penman’s Method |
| 1981-1986 |
701.47 |
1208.63 |
902.24 |
| 1987-1992 |
634.37 |
1029.50 |
784.25 |
Note: All the mean values are added by the standard errors.
Short-term water budget
The results of short-term water budget were shown in Fig.5. Monthly
evapotranspiration changed a bit before and after 1986; in April and October
evapotranspiration decreased on average. During these period agricultural fields become bare
soils for seeding and harvesting. Thus, evapotranspiration decreases for them. Therefore, the
decrease of evapotranspiration in April and October was contributed by agricultural activities.
That corresponds to the land cover change from forest to agricultural fields. Moreover, the
decrease of evapotranspiration seemed to correspond to withering paddy and grass.
Fig. 14 Monthly change of evapotranspiration
in the Doki river basin
4. Conclusions
From the satellite data in 1986 and 1990, 3 % of forest area was replaced to
agricultural fields. For 12 years including 1986, significant increase of runoff ratio was
examined with 0.05 of significant level. As the result of short-term water balance, for the past
12 years monthly evapotranspiration decreased in April and October for agricultural activities.
The increase of runoff ratio in the Doki river should be derived from the change of land use
from forest to agricultural fields.
Acknowledgement
We appreciate NASDA, Japan for providing Landsat TM data. A part of
this study was collaborated with NASDA, Japan as a project study named “Agriculture
Monitoring Using Satellite Remote Sensing”. This study was carried out under a cooperative
system for supporting priority research, which was implemented by Japan Science and
Technology Corporation.
References
- Jordan, C.F. & Heuveldop, The water budget of an Amazonian rainforest, Acta Amazonica, 11, 87-92, 1981.
- Ministry of Construction, River Bureau, Annual Rainfall Report, Japan River Association, 1981-1995.
- Ministry of Construction, River Bureau, Annual Discharge Report, Japan River Association, 1981-1995.
- Meteorological Agency, Monthly Meteorological Report, 1981-1992.
- Ministry of Agriculture, Forestry and Fisheries, Shikoku National Agricultural Experiment Station, Research
Materials, No.1,1988, and No.9, 1992..
- Suzuki, M., Evapotranspiration estimates from forest basin using short-term water budget method, Journal of
Japan Forest Society, 67,4,115-125,1985.
- Brutsaert, W.H., Evaporation into the Atmosphere, Kluwer Academic Pub., Boston, 1982.
- Kotoda, K, Estimation of River Basin Evapotranspiration, Environmental Research Center, University of
Tsukuba, 1986.
- Makino, I., S. Ogawa, G. Saito, Land cover change and discharge fluctuation in the Doki river,
Environmental Engineering Forum, in print, 1999.
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