Indicators in the study sites

Table of Contents

1. Compilers and contributors
2. Introduction
3. Climate indicators
4. Water indicators
5. Soil indicators
6. Vegetation indicators
7. Water runoff indicators
   1. Drainage density
   2. Flooding frequency
   3. Impervious surface area
8. Forest fires indicators
9. Agriculture indicators
10. Cultivation indicators
11. Animal husbandry indicators
12. Land management indicators
13. Land use indicators
14. Water use indicators
15. Tourism indicators
16. Human characteristics indicators
17. Institutional indicators
18. Conclusions
19. References

Drainage density

Drainage density is defined as the total length of the streams in a drainage basin divided by the area of the basin. Climate affects drainage density both directly and indirectly. The amount and the type of precipitation influence directly the quantity and character of runoff. In areas where precipitation comes largely as thunder showers a large percentage of the rainfall will run off immediately and more surface drainage lines will be formed. Furthermore, climate affects indirectly drainage density by the amount and kind of vegetation growing affecting surface water runoff. Drainage density is greatly affected by the infiltration capacity of the mantle rock or bedrock. It is commonly observed that drainage lines are more numerous over impermeable materials than over permeable ones. Drainage density is also affected by the initial relief or the vertical distance from the initial upland flats to the levels of adjacent graded valleys. In practice the necessary material to carry out this measurement is often not available, and only quantitative estimates can be made.

Drainage density (DD) has been measured for drainage basins in which field sites have been located. That means several study sites had the same value for drainage density. This indicator has been measured by using the most detailed material available such as topographic maps, aerial photographs, and satellite images. As Fig 40 shows, the length of drainage streams was measured and was divided by the total surface area of the basin. The following classes of drainage density have been used: (a) coarse, DD<5 km length per km² area; (b) medium, DD=5-10 km km^-2; (c) fine, DD=10-20 km km^-2; and (d) very fine, DD>20 km km^-2.

Fig. 40. Example of measuring drainage density on a basin scale

As Table 5 shows, drainage density has been measured in 911 field sites, corresponding to 11 study sites. The dominant class of drainage density class is coarse (DD<5 km km^-2), covering 39.5% (Fig. 41) of the total study field sites, corresponding to Secano Interior-Chile, Boteti Area-Botswana, Santiago Island-Cape Verde, Eskisehir-Turkey, Zeuss Koutine-Tunisia, Mamora, Sehoul-Morocco Novij Saratov-Russia, Cointzio Catchment-Mexico, and Crete-Greece study sites. The next important class of drainage density in the study field sites was fine (10-20 km km^-2), covering 33.4% of the total fields (Fig. 41). Such drainage density is prevailing in Guadalentin Basin Murcia-Spain, and Eskisehir-Turkey, and in some cases in Santiago Island-Cape Verde, and Crete-Greece sites. The next important the frequency of appearance of medium (DD=5-10 km km^{-2) and} very fine (DD>20 km km^-2) density classes were found in some cases covering 15.7% and 11.4% of the total study sites. Such drainage density classes were found in the study sites of Boteti Area-Botswana, Cointzio Catchment-Mexico, Santiago Island-Cape Verde, Novij Saratov-Russia, and Crete-Greece.

Fig. 41. Distribution of drainage density classes identified in the study field sites