Benefits of Watershed Management and Water Resources Management

The project directly finances physical infrastructure (Subcomponents 1.2 and Component 2) and supports soft interventions (Subcomponents 1.1 and 1.3) related to climate change adaptation and disaster risk mainstreaming. Climate co-benefits will be derived through (a) interventions to protect watersheds resulting in increased land area under sustainable landscape management practices and prevention of further degradation/sedimentation, and (b) increased area with improved irrigation resulting from better conveyance (gravity fed) and storage of water.

Benefits of Watershed Management

Restoring priority watersheds will have the larger benefit of maintaining the ecosystem and biodiversity. The ecosystem services of watersheds are complex and diverse and so are the benefits from watershed restoration. Some of the key quantified benefits expected are the following:

(a)

Improved irrigation from small tanks

Number of small tanks to be rehabilitated	8	Number
Average command area per tank	10	ha
Total command area of tanks	400	ha
Increase in net incomes	70,000	LKR/ha
Annual benefit of tank rehabilitation	0.03	US$, millions

Table 1: Benefits of Improved Irrigation from Small Tanks

(b)

Reduced electricity supply disruptions to change silt filters in hydroelectric plants on key dams especially during high silt-load monsoon flows.

If electricity breakdowns are reduced by 2 hours per year, because of reduced siltation in the river water (necessitating fewer shutdowns to clean or replace filters of the hydroelectricity turbines), all electricity consumers in the area will benefit not only from having power but also from not having to use expensive diesel for captive power generator sets. Assuming that there are around 10,000 diesel generating sets in the major establishments in the major towns (including shops, restaurants, hotels, and hospitals) in the catchment if such sets are used, and that each saves two hours per year, this provides an annual benefit of around US$0.02 million

Number of diesel generator sets in the Upper Mahaweli Watershed	10,000
Number of hours saved per year due to reduced silt	2
Annual hours of generation saved	60,000
Diesel consumed per hour (liters)	1.5
Cost of diesel (LKR per liter)	100
Annual benefit of improved electricity supply (US$, millions)	0.02

Table 2: Benefits of Reduced Electricity Shutdowns

(c)

Reduced drinking water supply shutdowns

The National Water Supply and Drainage Board supplies around 160,000 m³ per day through 15 water treatment plants in the watershed but has to stop operations when turbidity levels go over the treatment capacity (125 NTU). In 2018, turbidity levels were over 400 MTU for 7 months of the year (April–June and August–November). Although the plants were shut only during severe bouts of turbidity, disruptions of drinking water supply force urban consumers to buy water from private suppliers. Assuming that the plants worked at only 50 percent of capacity during the 7 months, this project reduces the days of service disruption by 15 percent and given that the price of buying water from private suppliers is around LKR 24 per liter, the total cost of turbidity-linked disruptions of water supply—and the benefit of reducing turbidity—is US$0.34 million per year.

Capacity of 15 river-side water treatment plants in the Upper Mahaweli Watershed	160,320	Cubic meters per day
Days when turbidity was higher than treatment capacity	210	Days (7 months)
Drinking water supply during days of high turbidity	50%
Annual quantity of water purchased privately during low supply	168.34	Million m³
Reduction rate of high turbidity days	15%
Cost of water from private suppliers	24	LKR/liter
Annual value of benefits of improved water supply	0.34	US$, millions

Table 3: Benefits of Improved Drinking Water Supply

(d)

Soil conservation on sloping seasonal crop land.

Using the nutrient replacement method for calculating the economic benefits of soil conservation, the costs of replacing the quantity of N, P, and K removed by soil erosion across this untreated area are US$0.66 million per year.

		On Site
	Price (LKR/kg)	Quantity (kg/ha)	Cost (LKR/ha)
N	242	53	12,826
P	268	388	103,984
K	187	7	1,309
Organic manure	1	653	653
Labor			540
Transportation	100		100
Total			119,412

Table 4: Unit Benefit from Soil Conservation

	Cost (LKR/ha)	Area (ha)	Value (US$, millions)
On-farm conservation	119,412	1,000	0.66

Table 5: Soil Conservation Benefits

Benefits of Water Resources Management

There are approximately 430 medium and large dams in Sri Lanka and over 12,000 small dams, most of which were built more than 1,000 years ago. Large reservoirs such as Kothmale, Victoria and Randenigala were built under the Mahaweli Projects. They are also now 30 years old and prone to cracks and leaks. This situation threatens the safety and operational efficiency of dams and their structures. If dam safety risks are not addressed before it is too late, the safety of lives, property and the environment could face serious threat.

Risks to the public and properties due to dam failures are minimized with the reduction of occurrence of dam failures through develop Operation and Maintenance Plans and mitigation measures and taking emergency actions to minimize risks by preparing Emergency Preparedness Plan if dam failures occur. The probability of occurrence of dam failures is being reduced by the ongoing DSWRP Project through various measures of rehabilitation of dams and structures such as riprap, spill and sluice and followed by carrying out Dams Portfolio Risk Assessment for the selected dams and classifying based on the Risk Index Scheme.

In order to minimize damages to the people and properties and dams due to critical situations, emergency action plans are being prepared for High-Risk Dams, whereby reducing the risk surrounding the downstream of the Dam and to protect the population at risk when failures occur.

Operation and maintenance (O&M) manuals are to be prepared for the dams where it classifies as high risk, to ensure the sustainability of the project works and ensure its safe operation. The following areas are discussed in O&M manuals, with respect to earth dams.

Direct benefits:

•	Secure about 357,000 families living under the 36 dams and 19 Schemes/Canals and their properties by increasing the safety of dams and structures.

Dam/Scheme	Storage Capacity (MCM)	Improved Command Area (Ha)	Beneficiaries (Families)
Victoria Dam	722	-	82,000
Randenigala Dam	861	-	3,600
Kotmale Dam	175	-	15,700
Rantambe Dam	21	18,595	34,171
Udawalawe Dam	267	19,499	41,000
Polgolla Barrage	4	-	350
Senanayaka Samudraya	950	48,342	52,300
Arawatta Tank	1.85	350	80
Peramadu Tank	3.0	250	287
Lunugamwehera Reservoir	226	5,466	10,000
Vahanery Tank	68	3,583	2,500
Bathalagoda Anicut	6	3,091	2,025
Mavilaru Tank	5	10,000	10,000
Dewahuwa Tank	14	1,215	2,025
Pavatkulam Tank	33.3	1,619	1,726
Nagadeepa Tank	29	607	5,000
Hali Ela Tank	3.85	100	350
Mahalindawewa Tank	4.5	184	140
Wadamunai Tank	5.30	148	121
Akathimurippu Tank	9	2,571	5,000
Kalmadu Tank	11.29	1,396	750
Akkarayan kulam	26.15	1,819	1,819
Kariyalai Nagapaduwan Tank	14.54	-	502
Mallavikulam	1.1	95	720
Welimaruthamadu Tank	3.41	607	700
Maruthankulam	2.810	182	725
Niththakaikulam	2.4	202.34	200
Periyamdu Tank	7.13	303	250
Piramanthalaru Kulam	3.86	602	201
Kudamuruty Tank	2.47	647	-
Paravipanchan Tank	5.80	219	200
Chadayantalawa Tank	3.95	1,134	1,150
Pulugunawai-Kankaniyar Tank	9.96	2,114	1,690
Adachchakal Tank	1.51	183	200
Meiyankal Tank	7.95	1,012	920

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