An Application of R. S. and GIS for Hydrological Modeling in Continental Scale
Ai Nakagawa, Shiro Ochi and Ryosuke Shibaski
Institute of Industrial Science, University of Tokyo
7-22-1, Roppongi, Minato-ku, Tokyo 106-8558, Japan
Tel: +81-3-3402-6231 Fax: +81-3-3479-2762
E-mail-: nakagawa@skl.iis.u-tokyo.ac.jp
Abstract
When thinking about food production to sustain world population, water resource is an important factor to define it. The quantitative analysis for available water is necessary to evaluate the cultivation potential which is enhanced by irrigation system. A hydrological model based on "Bucket Model" using Remote Sensing data and GIS is proposed in this paper to understand monthly distribution of water resource in continental map scale. When a precipitation is given to a pixel, the runoff from the pixel is calculated using "Bucket Model" in which the balance of Ep versus E (Evapotranspiration), and Sp versus soil wetness are considered. Monthly river runoff and soil wetness of each pixel with 0.5° grid are computed and simulated.
1. Introduction
The subject that securing enough food production becomes a very serious problem is pointed out from a global viewpoint. It is imagined that it is maybe impossible to expand agricultural area simply, so it is an important focus of discussion how far the expansion of agricultural productivity can be. Expected agricultural productivity depends on progress of technology; expansion of crop area or cultivation period by irrigation, increase of crops by improvement of species and wide advancement of agricultural land control or water control. Moreover, it is much influenced by deterioration of the soil etc. It is said that not only an artificial condition but also a natural condition of deterioration of the soil or availability of water resource etc. has much influence on the productivity. In this research, the availability of water resource of these condition is listened up and monthly available water is calculated in continental scale. Concretely, first, Ep(potential evapotranspiration) is calculated by using the temperature data presumed from remotely sensed data. And by using this, Et(evapotranspiration) and St(soil wetness in a pixel) are computed by hydrological modeling based on "Bucket Model". . In addition, the behavior of the runoff water is modeled by Distribution Model, and monthly river runoff and monthly available water of each pixel.
2. Construction of the Model
2.1 Outline of the Model
When it rains, the part of rain water is intercepted by leaves of the trees or the crop grasses while the other part falls to the ground directly. A part of the rain water which falls to the ground floods to the earth and the remainder gathers to the lower land. Rain water stocked in the low ground increases it depth because of the continuance of the rainfall and finally flows out to lakes and marshes and oceans as a stream. The rain water which flows out to the ground infiltrates in perpendicular direction. When it reaches the layer which water easily permeates, it flows horizontally along the layer and flows out to the ground or the river on the way. Moreover, water evaporates from leaves, the ground, and the surface of the water and spreads into the air. In this research, the mechanism is simplified as follows. The behavior of water from falling to the ground to runoff is represented by "Bucket Model" in which the balance of Ep versus Et, and Sp(potential soil wetness in a pixel) versus St are considered. And the behavior of runoff water is represented by distributed model in which inclination of the ground is considered.
2.2 Bucket Model
This model supposes that water runs off only when the amount of the rain fall of one month and the water stock unil last month having subtracted the evapotranspiration exceed the potential water stock. (Figure 1).
Figure 1. Bucket Model
The relation between Ep (potential evapotranspiration), Et (evapotranspiration), Sp (potential soil wetness) and St(soil wetness) is assumed such as Figure 2. This shows, that evapotranspiration occurs only when some ground water exists, and that evapotranspiration from ground becomes constant and he surplus runs off when the wetness of the land exceeds the potential water stock.
Figure 2. the relation between Et/ Ep and Et /Sp
2.3 Distribution Model
Distribution model used at this time assumed that the runoff from a certain pixel flows only to the pixel in the surface flow direction which is made from digital elevation model (DEM). Further, the amount of the runoff from each pixel is by the Bucket described in the foregoing paragraph.
