Chapter 1.1.5: Key Problems and Challenges in Watershed Management 1

Module 1: Basics of Watershed Management Technical Annex

Delineation of Watersheds This technical annex illustrates the way in which a drainage unit such as a watershed or catchment can be practically delineated. A concrete example is given from the Svay River catchment in Pursat Province in Cambodia and so a topographic map with a scale of 1:50,000 was used to determine the boundaries of the Svay River catchment [3].

The watershed divide is a line, on either side of which the water flows to different rivers, and therefore it forms the watershed boundary. During the watershed delineation process this watershed divide is identified and marked on a map. It is a line connecting the highest terrain points between two neighbouring rivers and is represented in the landscape by topographic features such as crests and mountain ridges. Contour lines are used to guide the drawing of watershed boundaries.

The delineation process in this example is illustrated with a series of maps which include brief descriptions of related activities. The red line in the maps indicates the catchment boundaries.

Beside topographic maps, GIS and digital elevation models can be used to perform computer based watershed delineation and these apply different GIS systems and custom applications ([1], [2]). These methodologies use precisely the same principle which has been introduced in this example.

References and Sources for Further Reading [1] Djokic, D.; Ye, Z. 2006: DEM Preprocessing for Efficient Watershed Delineation. ESRI Redlands, California.

http://gis.esri.com/library/userconf/proc99/proceed/papers/pap676/p676.htm

[2] EPA, 2007: Automatic WS Delineation. http://72.14.235.104/search?q=cache:3Qr2ppC2VkEJ:www.epa.gov/waterscience/basins/b3docs/sec8_2.pdf+watershed+delineation,+definition&hl=en&gl=la&ct=clnk&cd=2

[3] Kirsch, H. 2004: Watershed Delineation – A Practical Example from Cambodia. [unpublished]

Chapter 1.1.5: Problems and Challenges in Watershed Management 2

Figure 2: The border delineation begins at the confluence of the Svay River and the Thriek River. In the plain the boundary is drawn between the Svay River and the O Tala River and moves into a northern direction towards the Damrei Slap mountain ridge.

Figure 1: The boundary continues in a northerly direction and follows the lower mountain ridges of the Damrei Slap towards the Chruk Tukay mountain ridge.

Chapter 1.1.5: Problems and Challenges in Watershed Management 3

Figure 4: The boundary continues along the mountain ridge of the Roleap Samrav, up towards a hill in the northern part of the catchment, and then turns in a westerly direction.

Figure 3: The boundary then shifts and heads in a southerly direction following the ridges of the Kde and Damak mountains.

Chapter 1.1.5: Problems and Challenges in Watershed Management 4

Figure 5: It continues further south following the low

er ridges of the Dam

ak Mountain

and then finally ends back at the confluence of the Svay R

iver and the Thriek River.