Keywords: agricultural fields, deforestation, evapotranspiration, Landsat TM, runoff ratio
Abstract
Land cover change and discharge fluctuation for the past 12 years in the Doki River
catchment were examined using multi-temporal satellite data. This catchment is composed of
mainly forest and some agricultural fields along the streams, the ratio of which is more than
10 % of the total area. Using the multi-temporal satellite data, Landsat TM, in 1986 and 1990,
approximately 3 % of the total area had changed from forest to the agricultural fields during
this period. From the water budget from 1981 to 1992 in the catchment, evapotranspiration
decreased and runoff ratio increased. Short-term water budget led to monthly
evapotranspiration before and after 1986, which showed the evapotranspiration in April and
October decreased. Such change means agricultural activities, seeding and harvest. Moreover,
statistical tests for hydrological data before and after 1986 showed that runoff ratio increased
significantly with 0.05 of significant level. Therefore, one of the factors that caused the
increase of runoff ratio in the catchment should be the land cover change from forest to
agricultural fields.
1. Introduction
Land cover changes in the basin affect on its discharge and bring not only flood but
also hard conditions to get agricultural and urban water supply. Especially, land use changes
to agricultural fields or golf courses by deforestation have been paid attention as an important
environmental change in the same way as urbanization recently (e.g. Jordan & Heuveldop,
1981). Then, land cover change and its effect on long-term water budget in the Doki river basin
in the western part of Japan were examined from its satellite data and hydrological data. The
effect of small development for land cover was evaluated with the long-term trends of water
budget and the seasonal profiles of monthly evapotranspiration.
2. Materials and Methods
An objective site is located in the Doki river basin in Shikoku Island, Japan. The
satellite data of multi-temporal Landsat TM (Table 1) were used for calculation of land cover
classification, its area ratios, and NDVI.
DEM, 25,000 and 50-m mesh, was used for the watershed separation. Hydrological
data were precipitation and discharge in the Doki river observed for 12 years by Ministry of
Construction. Discharge observation sites are located at Goyo Bridge (Drainage area: 72.1
km
2 ) and Tsunekane Bridge (90.0 km
2 ), while precipitation observation site is located at Miai
at upper stream of the Doki river watershed as shown in Fig. 1. Meteorological data were used
for evapotranspiration. The meteorological observation sites are at Tadotsu Meteorological
Observation Station and Shikoku National Agricultural Experiment Station in Ministry of
Agricultural, Forest and Fishery.
Basin boundaries were determined semi-automatically by TNTmips, GIS software,
from DEM with 50-m mesh. The boundary that intersects the discharge observation site was
determined with geomorphologic maps considering micro drainage areas.
Annual evapotranspiration was calculated from annual precipitation and discharge for 12 years in
the Doki river basin using Equation (1). Water balance was calculated from monthly rainfall and
discharge using the next equation to get evapotranspiration. Also runoff ratio was obtained
from r=Q/P.
where S is the storage, P the rainfall, Q the discharge, and E the evapotranspiration. Moreover,
monthly evapotranspiration was also calculated with short-term water budget method.
Daily evapotranspiration each land cover was derived from meteorological data using
the Penman equation (Brutsaert, 1982).
where
D=(de*/dT)
T,
lº C
pP/
el,
Q
n is the available energy flux density in mm/day, f(u)
the wind function in mm/day, u the mean wind speed in m/sec, e
a* the saturation vapor
pressure in hPa, e
a the vapor pressure in hPa,
R
n the net radiation in W/m
2 , L
l the latent heat of
vaporization (=2.454.10
6 J/kg), R
s the short-wave radiation in
W/m
2,
as the albedo of the
surface, R
ld the downward long-wave radiation in W/m
2 ,
R
lu the upward long-wave radiation in
W/m
2 ,
es
the emissivity of the surface (=0.97),
ea
the atmospheric emissivity,
a the
Stefan-Boltzmann constant (=5.67*10
-8Wm
-2K
-4),
T
a the air temperature in K, and T
s is the
surface temperature in K.Here, albedo each land cover was referred from empirical values
(Kotoda, 1986).
Land cover classification was carried out from Landsat TM with the maximum
likelihood method to obtain 7 land covers: forest, paddy fields, vegetable fields, bare soils,
urban areas and water body, and their area ratios. The ground truth was confirmed for land
use in June 1998 and February and June 1999.
Fig. 1 Doki River and its observation stations
Table 1. Satellite data used. From 1986 till 1990,
agricultural development was carried out.
| Satellite |
Date |
| LandsatTM |
1986/4/12, 9/3, 11/6.1990/3/22, 5/9 |
