Assessment of crop productivity for major river basins in asia
Using gis and rs data
3. Methodology and Output
3.1 Estimation of Net Primary Productivity
Monthly Net Primary Productivity(NPP) can be estimated using NDVI and PAR data by "production efficiency approach" proposed by Goward and Huemmrich(1992) and Ruimy, Saugier and Dedien(1994).
NPP=e ò ¦APARdt .........(1)
NPP:[gDM/m2/time],
e : efficiency[g/MJ],
¦APAR:Function of Absorbed PAR [MJ/m2]
¦APAR=-0.08 + 1.075xNDVI ..........(2)
e is 1.5 for global average.
The annual total NPP can be estimated as follows:
By using this formula, annual total of Net Primary Productivity(NPP) from 1981 to 1989 was generated. The output was verified using existing study concerning the global NPP and the regional NPP. Table-1 and Table-2 show the comparison of Global NPP versus our study and Regional NPP versus our study respectively(Ochi and Murai,1999). By comparing with other studies, the adopted method in our study for estimation of NPP is acceptable for the purpose of our study goal, though there have been no reference data.
Table -1 Verification of Global NPP
| Model |
GNPP(g ton DM/year) |
| Whittaker(1979) |
115.6 |
| Lieth(1977) |
106.0- 125.8 |
| Box(1989) |
122.9 |
| Gotoh(1993) |
110.2 |
| Ochi/ Murai(1999) |
119.4 |
Table-2 Verification of Regional NPP
| |
NPP(g ton) |
| Africa |
22.9 |
| North/Centeral America |
19.1 |
| South America |
30.3 |
| Eurasia |
38.6 |
| Oceania |
8.5 |
| Total |
119.4 |
3.2 Correlation between Global NPP and Cereal Production
Figure-2 shows the relationship between Global NPP derived by this study and world Cereal Production prepared by FAO. The correlation factor(R) of the two items is 0.91 in the period from 1982 to 1990. However, the correlation become worse when the data from 1991 to 1993 are involved. It is said that the NDVI data and PAR data include some noise due to the aerosol caused by Mt. Pinatubo Eruption of 1991 therefore the data after the eruption are not adequate for the use. It was found that the NPP can be used for the estimation of crop(or cereal) production.
3.3 Change of NPP in major river basins in Asia
By overlaying USGS/IGBP cover map with global NPP map, NPP from each land cover category can be extracted. Figure-3 shows that changes of NPP from 1982 to 1990 in major six river basins in Asia ; Amur,
Figure-2 Correlation between NPP and World Crop(Cereal) Production
Figure-3 Changes of NPP in major river basins in Asia
Yellow, Yangzhu, Mekong, Ganges and Indus. NPP are mainly produced in the crop and grass land with high population density in the Yangzhu, Yellow, Ganges and Indus river basins. On the contrary, NPP are produced in the forested area in Amur and Mekong river basins where population density is not so high. NPP of crop land are more stable than NPP of other land covers.
