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Poster Session 5
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Land Cover Change and its Effect on Runoff in the Doki River Catchment
3. Results and Discussion
Land cover change
Land cover area ratios are shown in Table 2 from the land cover classification of the
Doki river basin in 1986 and 1990. Comparing 1990 with 1986, about 3 % of forest area
decreased instead agricultural fields increased by the same ratio. Here, considering land use,
agricultural fields change as paddy fields, vegetable fields, bare soils, and grass seasonally, and
therefore these categories should be unified to one land cover. Land cover classification maps
in 1986 and 1990 are shown in Fig. 2, where conventionally forest, agricultural fields and
others are indicated. In total, a part of forest in 1986 was changed to agricultural fields by
1990. This part equals about 3 % of the total basin.
Table 2. Land cover change in the Doki river basin using Landsat TM (Unit: %). From 1986 till
1990, 3 % of forest was developed to agricultural fields. At the same time, 0.5 % of
paddy fields were changed to vegetable fields.
| Year |
Forest |
Paddy Field |
Vegetable Field |
Bare Soils |
Grass |
Urban Area |
Water |
| 1986 |
88.4 |
6.1 |
4.2 |
0.0 |
0.0 |
0.7 |
0.6 |
| 1990 |
85.5 |
5.6 |
6.2 |
1.1 |
0.2 |
0.9 |
0.5 |
Fig. 2 Land cover classification in the Doki river basin using Landsat TM in 1986 and 1990.
Long-term water budget change
The results of water balance from 1981 till 1992 are shown in Table 3. From annual
precipitation and discharge from 1981 till 1992, annual evapotranspiration and runoff ratio
were calculated. During this period including 1986, annual precipitation indeed increased,
but annual discharge increased more than the increase rate of precipitation as shown in Fig. 3.
On the other hand, annual evapotranspiration increased at the Goyo Bridge, but decreased at
the Tsunekane Bridge. Annual runoff ratio increased with 0.05 of significant level. As shown
in Fig.4, annual runoff ratio was the minimum in 1985, and had peaks in 1990 and 1993. The
latter peak seems to be affected by golf course development (Makino et al., 1999). In total,
annual runoff ratio increased monotonously from 1985 till 1990. Moreover, including the total
observation period from 1970 till 1995, annual runoff ratio increased. Since annual
precipitation shows almost constant, the increase of runoff ratio may be affected by the
independent factor from meteorological factors.
Table 3. Change of rainfall, discharge, evapotranspiration, and runoff ratio in the Doki river
basin (Unit: mm).
| Period |
Rainfall |
Dis-
chargeG |
Dis-
chargeT |
Evapora-
tionG |
Evapora-
tionT |
Runoff RatioG |
Runoff RatioT |
| 1981-86 |
1122±67 |
377±70 |
422±53 |
745±39 |
701±47 |
0.33±0.05 |
0.37±0.04 |
| 1987-92 |
1412±116 |
632±121 |
778±135 |
780±51 |
634±37 |
0.43±0.05 |
0.53±0.05 |
Note: Subscripts G and T represent Goyo Bridge and Tsunekane Bridge observatory stations. Runoff ratios are dimensionless.
Fig.3 Time series of rainfall, discharge and
evapotranspiration in the Doki river basin
(1986-1995).
Fig. 4 Runoff ratio in the Doki river basin(1986
-1995). Runoff ratio increased during
the observation.
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