Watersheds nearly always part of a large watershed. The subdivision of the watershed into the smaller unit is known as ‘Sub-watershed’. The watershed collectively forms a larger subbasin and basin. All the watershed units don’t receive equal attention for management and conservation activities. The prioritization of sub-watersheds based on their present condition, the extent of degradation, and sensitivity, and therefore the study of sub-watershed has great significance in watershed management planning. To effectively prioritize sub-watershed, a careful interpretation of each sub-watershed characteristics property related to the area. The morphological parameters used for sub-watershed prioritization include area, perimeter, stream order, mean stream length, etc. Only viable techniques are applied for the prioritization of sub-watershed.
 |
| Sub-watershed |
Sub-watershed Prioritization
Sub-watershed prioritization is the ranking of different sub-watershed of a watershed according to the order in which they have to be taken for treatment and soil conservation measures. Implementing land use practice and water management practices to protect and improve the quality of water and other natural resources within a watershed by managing the use of those land and water resources in a comprehensive manner is watershed management. Management of larger watersheds is difficult to achieve at once thus, the concept of sub-watershed prioritization comes into existence to proceed with conservation one after another based on its priority and necessity.
Why sub-watershed prioritization?
Sub-watershed is prioritized based on the availability of natural resources, socio-economic conditions of locals, including drainage density, slope, water yield capacity, groundwater prospects, soil, wasteland, irrigated area, forest cover, population, rainfall, agriculture laborer’s, etc., and other based on the local condition for priority and necessity. Sub-watersheds help to conserve natural resources on a sustainable basis with proper development and management plans which ultimately lead to soil and water conservation.
The human induces constraints for prioritization of sub-watershed are:
- Limited resources (Human and Financial)
- Huge demand from every corner.
- No impact when resources thinly distributed.
- Concentrate activities within a reasonable geographical area.
Data used for sub-watershed prioritization
Sub-watershed can be prioritized using geomorphometric parameters like bifurcation ratio, drainage density, steam length, compactness coefficient, stream frequency, texture ratio, length of overland flow, form factor, circularity ratio, and elongation ratio, etc. Sub-watershed analysis and prioritization can also be carried on using high-resolution images (at least 30 m resolution) as primary data through the GIS Approach. Analyzing the information like land system and land use data from the map and demographic data from the watershed helps in prioritizing the sub-watershed. Here, in this practical session, we would use this methodology to rank the sub-watershed based on its priority for conservation and erosion control. The used data are:
- Land System Data
- Land Use Data
- Population Data (population density = population of SW / SW area)
Land System:
Nepal is sub-divided into five ecological zones inland system map (1:50,000) published by Land Resource Mapping Project (LRMP). The ecological zones are Terai, Siwaliks, Middle Mountain, High Mountain, and High Himal classified based on physiography, geology, and geomorphology. Based on the recurrent pattern of landforms, geological materials, slopes, and agriculture limit of 17 land systems and landforms mapped. And based on landscape features such as position slopes, degree of dissection, flooding frequencies, soil characteristics (drainage, depth, texture, profile development, and pH), divided into 43 land units.
Land units denoted by the combination of digits and alphabets in the land system map.
Ex. 2b – Terai, active alluvial plains, intermediate position. (1-3 represent Terai)
- Ecological zone (5)
In land system maps, the label/notation with the numerical part represents Ecological zone as shown: Terai (1 - 3), Siwaliks (4 – 8), Middle Mountain (9 – 12), High Mountain (13 – 15) and High Himal (16 & 17).
- Land system/ Land form (17)
- Land units (43)
Land Use
Nepal is broadly classified based on land use in the land utilization map (1984). The land uses is denoted using different notation for different land use.
Notations used on the map (land utilization map) are as mentioned:
- Hillslope cultivation: Level terrace (T), Sloping terrace (C) (3 sub-classes)
- Valley cultivation: Valley floods (V), Tars and fans (F), Grazing land (G), shrub Land (S)
- Forest land: based on forest types and crown density – 4 sub-classes
Ex.
- C3 for stands in sloping terraces with more than 75% cropping intensity.
- Forests (3) for Forest with crown density 40-70%.
- Grazing – (G), Shrub (S).
Procedure for the prioritization of sub-watershed
Step 1: Determine Land System Erosion Potential (LSEP)
From the land system maps, each land unit are classified into three categories high (h), medium (m), and low (l) depending on land system erosion potential. While categorizing landscape features like position, slopes, degree of dissection, flooding frequency, and soil characteristics like drainage, depth, texture, profile development, pH of each land unit are taken into consideration.
Ex:
- 4a (h): Siwaliks region, active and recent alluvial plains, sand and gravel bars
- 9b (l): Middle mountain region, alluvial plains, and fans, alluvial plain
- 10b (m): Middle mountain region, ancient lake and river terraces, dissected.
Step 2: Determine Land Use Erosion Potential (LUEP)
From the land utilization maps, each land use is classified into three categories high (H), medium (M), and low (L) depending on land use erosion potential.
Ex:
- C3 (H) – Hill slope cultivation, sloping terraces, cultivation intensity more than 75%
- T1 (L) – Hill slope cultivation, level terrace, cultivation intensity 15-50 %
- V (M) – Valley cultivation, valley floods.
Step 3: Determine Land Use Land System Erosion Potential (LULSEP)
In this step, combining both Land System Erosion Potential and Land Use Erosion Potential we can find out the Land Use Land System Erosion Potential of an area or a sub-watershed.
LULSEP = LSEP + LUEP
When the erosion potential categories from two different maps combined, then denoted with single notation reflecting combined erosion potential. The combined erosion potential has mainly five categories namely very high, high, moderate, low, and very low. They are categorized as shown.
|
Land Use Erosion
Potential |
Land System Erosion
Potential |
||
|
|
High (h) |
Medium (m) |
Low (l) |
|
High (H) |
Hh = H |
Hm = h |
Hl = M |
|
Medium (M) |
Mh = h |
Mm = M |
Ml = L |
|
Low (L) |
Lh = M |
Lm = L |
Ll = l |
|
H = Very High, h =
high, M = Moderate, L = Low, l = very low |
|||
Step 4: Calculate Land Use Land System Erosion Potential Value (LULSEPV)
Using, the combined erosion potential categories LULSEP, we can calculate land use land system erosion potential value. While calculating the LULSEPV, it is considered that the erosion values differ for different erosion potential categories. So, while determining erosion values area of very high erosion potential is multiplied by 8, high erosion potential area by 6, moderate erosion potential area by 4, low erosion potential area by 2, and very low erosion potential by 1.
The formula for calculating LULSEPV is;
LULSEPV = (area of very high Land Use Land System Erosion Potential * 8 + Area of high LULSEP * 6 + Area of moderate LULSEP * 4 + Area of low LULSEP * 2 + Area of very low LULSEP) / Total area of the sub-watershed.
Step 5: Calculate Sub-watershed Population Density (SWPD)
Population data is important to calculate the difference in the erosion in the presence and absence of population and also to find out by how much amount the erosion differ with various population size. This helps to determine the influence of human presence on erosion. Population data from Census data or records keep by government agencies of the area or the sub-watershed is used. The total population of that area divided by the area of that location gives the population density of that area/location/sub-watershed.
The formula used is:
SWSPD = SWS population /SWS area
where SWS stands for Sub-WaterShed.
Step 6: Normalization of LULSEPV and SWPD
In this step, for the normalization of LULSEPV and SWPD values, the largest value of LULSEPV divides all other values of LULSEPV and the largest value of SWPD divides all other values of SWPD. This helps to compare differences in erosion potential values with respect to areas with highest erosion potential values and areas with the highest population density.
Note: Ignore the high population of urban centers.
Step 7: Giving weightage to LULSEPV and SWPD
More weightage is given to LULSEPV value i.e., 60% while only 40% weightage given to SWPD values.
LULSEPVweightage = LULSEPV * 0.6SWSPDweightage = SWSPD * 0.4
Step 8: Adding LULSEPVweightage and SWSPDweightage
Adding LULSEPV and SWSPD after weightage values to get the sum value. This sum value is used to rank the watershed based on prioritization. Higher the value higher the priority and hence rank ahead of others.
Total value = LULSEPVweightage + SWSPDweightage
Step 9: Rank the Sub-watershed
The total with the highest value gets rank 1 and rank 2, 3, 4, … given respectively to the sub-watershed with the values below the highest value respectively. The sub-watershed with the rank 1, 2,3, … are considered sub-watershed with high priority to low priority sub-watershed respectively. The prioritized sub-watershed is regarded as; they are at high risk of erosion, land degradation.
Example demonstration according to the steps shown above. Here in the table below begins with LULSEP values and SWSPD values already calculated.
 |
| Normalized values, weightage, calculate rank |
Ranking the prioritization of sub-watershed;
 |
| Ranking of subwatershed on priority based. |
0 Comments