A case study of the Mahaweli Area in Sri Lanka

The Mahaweli Development Programme is a water resource-based multi-purpose development project in Sri Lanka and the Upper Mahaweli Catchment (UMC) in the Central Highlands receiving a high amount of rainfall. This paper based on secondary data examines the relationship between rainfall, relative humidity, stream flow of the UMC namely and paddy production at Mahaweli.

The secondary data used in this study were from the Department of Meteorology, and the Hydrology Division of Irrigation Department. Seasonal variation of monthly rainfall and relative humidity during day and night studied. Variation of seasonal rainfalls at Kandy using moving averages studied and indicated for, northeast monsoon (NEM), first inter monsoon (FIM), southwest monsoon (SWM) and second inter monsoon (SIM). Also the annual rainfall trend at Kandy studied.  Trend of runoff at Peradeniya from 1944 to 1980 and to 1997 was calculated using regression analysis.

Statistical analysis  use to select dependent variable stream flow of the Mahaweli Ganga during n^th  month at Peradeniya (SFn)  of Ln (SFn), with independent variables. The stream flow of the Mahaweli Ganga during n^th month at Peradeniya (SFn) five regression equations are developed.          Monthly data of gauging stations from 1944 to1980 period are considered in this correlation analyses and regression analyses. Graphic presentations, correlation analyses and regression analyses were also performed.

The average annual rainfall at all selected stations reduced from 1931-60 period to 1961-90. The percentage of rainfall reduction rate varies from 2.92 to 18.02 mm/year. The average monthly rainfall at Katugastota reduced with 0.497 mm/month/year and average stream flow of Mahaweli Ganga at Peradeniya increase with rate of 0.15817 cubic meters/second/year.

The stream flow of Mahaweli Ganga at Peradeniya n^th month (SFn in 1000 cubic meters), calculated rainfall at Kandy during the n^th month (RFn in mm), rainfall at Kandy in (n-1)^th , (n-2)^th and (n-3)^th month are RFn-₁,  RFn-₂  and  RFn-₃ respectively and average n^th month relative humidity during the daytime and night time at Kandy are RHDn and RHNn respectively the stream flow is given by

Ln(SFn)  = 9.651741+ 0.003296RFn + 0.000808RFn-₁  – 0.00019RFn-₂ – (8.9E-05)RFn-₃ 

                                    +   0.083092RHDn  – 0.13583RHNn  

Stream flow (SFn) during southwest monsoon (SWM ), northeast monsoon (NEM), first inter monsoon (FIM) and second inter monsoon (SIM) seasons are given by (2) to (5)equations

The highest duty 3.39 m at System C during 1999 Yala season and the lowest duty 1.22m at System H during 2000 Maha season. The water productivity shows the highest value of mean water productivity is at System H and the second highest at System C. System H shows the highest water productivity value (0.39 Kg/ m³) and the lowest value (0.11 Kg/m³)at System B.

Trend of national and Mahaweli paddy production within this period shows 26,692 and 18,536 Mt/year increasing trend respectively.  The study shows increasing trend of OFC, banana and vegetable production from 1997 to 2005. 

Introduction

Climate change has been heralded as a threat to the world and especially to developing countries. The latest reports of the Intergovernmental Panel on Climate Change (IPCC) quote a 0.76⁰ C increase in the world’s average temperature in the last century, expecting temperatures to increase by 2⁰ C by 2050 (Chavez et al.2008). The IPCC’s research findings generated the momentum for the foundation of the United Nations Framework Convention on Climate Change (UNFCCC) and later the Kyoto Protocol. Sri Lanka ratified the UNFCCC in 1993 and the Kyoto Protocol in 2002 (UNFCCC 2003). In tandem with these ratification processes, studies were conducted to assess the potential climate change impacts on Sri Lanka. Subsequently, Sri Lanka developed policy recommendations on the basis of UNFCCC guidelines that addressed the need for the nation to engage in climate change mitigation and adaptation measures (Mahaweli Authority of Sri Lanka 2000).

In contrast, adapting to climate change may be more important for Sri Lanka. Being a small island nation, Sri Lanka falls into the UNFCCC and IPCC’s category of ‘vulnerable’ small island nations under serious threat from various climate change impacts, such as sea level rise and severe floods and droughts (UNFCCC 1992; IPCC 2001). These threats are considered to have significant negative consequences on various sectors within Sri Lanka.

Sri Lanka covers about 6.56 million hectares of land area which 1.8 percent include inland waters. In 1871, when Sri Lanka contained only 2.4 million people, about 2.7 hectares were available per person. Today at about 20 million, land has decrease to 0.34 hectares per person.  Population increases have significantly changed land uses since independence, when high growth rates intensified competition for land in Sri Lanka. Very important changes in land use, during this century have been documented. For example in 1881, forest was estimated to have covered 81 percent of land in Sri Lanka and by 1900 they were reduced to about 70 percent (IMPSA 1992). Since 1956 natural forests shrinks from 2,900,000 hectares to 2,150,000 hectares in 1984. Assume against population increases, forest declined from 0.35 to 0.13 hectares of forest per head. New irrigated lands brought over 300,000 families into the Dry Zone including settlement of 51,000 hectares of the Mahaweli region, where 10,000 people moved of the Mahaweli reservoirs (NARESA 1991).

Paddy cultivation increases from 514,000 hectares to 760,000 hectares in 1988, mainly in the newly irrigated areas of the Mahaweli. Annexed Table 2 indicate information on new reservoirs of Mahaweli Ganga Scheme. Sugarcane which barely existed in 1956, increased to about 20,000 hectares with the opening of plantations in the Southern and Eastern dry zone. Tea plantation declined in size by about 10 percent, but the overall acreage of developed agricultural land- about 30 percent in 1982 barely changed.  The Land Commission estimates a 10 percent coconut decline between 1962 and 1982.

The 335Km long Mahaweli river traverses from the so-called "Wet Zone" of Sri Lanka to the "Dry Zone" - a process which has been augmented with dams, canals and tunnels starting over 2000 years ago. The Accelerated Mahaweli project, which was carried out in the last two decades, was the largest development project in Sri Lanka involving the generation of hydroelectricity, irrigation of the "Dry Zone", land settlement, employment generation and infrastructure development. This project provides irrigation water to additional 365,000 ha of land in the Dry Zone. Its catchment spans 10,448 square km and it feeds 1003 tanks. Subsequent to construction, attention has shifted to watershed management, water management, crop diversification, participatory management and enterprise development. There have been concerted efforts to manage the watershed in the upper reaches of the Mahaweli.

Annual average of rainfall of Sri Lanka has been decreased by an amount of 144 millimeters, about seven percent, during 1961 to 1990 period compared to 1931 to 1960 period (Chandrapala, 197) with the standard deviation increasing from 234 to 263 millimeters. Northeast monsoon rainfall over Sri Lanka has been decreased from 1931 – 1960 to 1961-1990 periods.

Annual mean air temperature anomalies have shown significant increasing trends during the recent few decades in Sri Lanka (Basnayake et al 2002). The rate of increase of mean air temperature for the 1961-1990 period is in the order of 0.016 ⁰C per year (Chandrapala, 1997). Annual mean maximum air temperatures have shown increasing trends in almost all stations with the maximum rate of increase about 0.021 ⁰C per year at Puttalam (Basnayake, 2002). Nighttime annual mean minimum air temperatures have also shown increasing trends with higher gradients. The maximum rate of increase of nighttime annual mean minimum air temperature is reported about 0.02 ⁰C per year at Nuwara-Eliya (Basnayake, 2002).

The reduction rates of tea lands at Kotmale, Victoria, Randenigala and Rantambe are 31.07%, 70.90%, 50.2% and 32% respectively from 1956 to 1992. During the same period forest cover in these sub-catchments has correspondingly increased by 15.79%, 224.48%, 61.54% and 96.94% respectively. Bad land use practices promote soil erosion in this sloppy lands in four catchments and water capacity of reservoirs reduced dramatically. In lowland Sri Lanka, the mean minimum temperature does not generally fall below 21 ^OC in any month of the year. However, it decreases markedly in the highlands with increasing altitude. At Nuwara Eliya, the mean minimum temperature in January, February and March is 9.4, 9.5 and 10.2 ^OC respectively (Jayatillake et al, 2005).    

Methods

The secondary data used in this study were from the Department of Meteorology, Department of Census and Statistics, National Water Supply and Drainage Board and the Hydrology Division of Irrigation Department. Compare 30 year averages (1931-60, 1961-90) of rainfall and temperatures at the selected stations within the Mahaweli Project Area

Seasonal changes and times series analysis of data were used for trend analysis. Seasonal variation of monthly rainfall and relative humidity during day and night studied. Variation of seasonal rainfalls at Kandy using moving averages studied and indicated for, northeast monsoon (NEM), first inter monsoon (FIM), southwest monsoon (SWM) and second inter monsoon (SIM). Also the annual rainfall trend at Kandy studied.  Trend of runoff at Peradeniya from 1944 to 1980 and to 1997 was calculated using regression analysis.

Correlation analysis  use to select dependent variable stream flow of the Mahaweli Ganga during n^th  month at Peradeniya (SFn)  of Ln (SFn), with independent variables. The stream flow of the Mahaweli Ganga during n^th  month at Peradeniya (SFn) then the regression equation is written as.

Ln(SF n)  = α₀ +     α₁RFn+      α₂RFn-₁ +    α₃RFn-₂  +     α₄RFn-₃  + α₅  RHDn+ α₅    RHNn 

SFn     - Peradeniya Stream flow n^th month in cubic meters per second

RFn     -  Kandy rainfall during n^th month in mm

RFn-₁  -  Kandy rainfall during (n-1)^th month in mm

RFn-₂ -  Kandy rainfall during (n-2)^th month in mm

RFn-₃ -  Kandy rainfall during (n-3)^th month in mm

RHDn   -Average daytime relative humidity Kandy during n^th month as %

RHNn  -Average nighttime relative humidity Kandy during n^th month as %

Monthly data of gauging stations from 1944 to1980 period are considered in this correlation analyses and regression analyses. Graphic presentations, correlation analyses and regression analyses were also performed.

Water issues, water duty and water productivity for Mahaweli System B, C, G and H analyzed up to year 2000 (Annexed Fig 1). Also water issues, water duty and water productivity variation of System B,C, G and H analysis.

Results

Average annual rainfalls of selected meteorological stations are indicated in Table 1.  According to the table at all stations the average annual rainfall reduced from 1931-60 period to 1961-90. The percentage of rainfall reduction from 1931-60 to 1961-90 period is highest at Matale and second highest at Kandy and the lowest at Vavuniya. This is bad situation because main three reservoirs are close to Kandy and if rainfall reduce in future then it may be badly effect not only agricultural production but also to hydro power production due to lack of water storage. Also at the Mahaweli areas rainfall reduction directly effect on rain fed farming and lands under minor and major irrigation schemes. Amount of irrigated water release for cultivation able to reduce if sufficient rainwater received by the area. Cultivations at Matale, Anuradhapura, Batticaloa and Polonnaruwa may be seriously affected in future because percentages of rainfall reductions are 18.02, 9.103, 8.212 and 7.638 respectively.

Table   1  Average annual rainfalls of  selected meteorological stations

Districts	Annual AVG Rainfall		AVG Rainfall Change	% of rainfall reduction
	D1 1931-1960	D2 1961-1990	( D2-D1)	(D2-D1)*100/D1
Kandy	2693	2375	-318	-11.81
Matale	2369	1942	-427	-18.02
Nuwara Eliya	2848	2638	-210	-7.374
Mannar	1230	1140	-90	-7.317
Vavuniya	1440	1407	-33	-2.292
Batticaloa	1790	1643	-147	-8.212
Trincomalee	1703	1587	-116	-6.812
Anuradhapura	1505	1368	-137	-9.103
Polonnaruwa	1833	1693	-140	-7.638

Source :Department of Meteorology  1975 and 1992

The average monthly rainfall variation 1961-90 indicated in the Figure 1 shows a very low amount of average rainfall during the southwest monsoon (May-September) in  dry zone stations Anuradhapura and Maha Illuppallama. Hence water scarcity is the main constrain for Yala cultivation in the dry zone from May to September. During the same season Kandy and N’Eliya within the UMC received a high rainfall. This water is diverted to the Mahawelis as well as other areas for cultivation.

Source : Department of Meteorology  1992

Figure 1: Seasonal Variation of AVG Rainfall (1961-1990)

Variation of average monthly rainfall (mm) at Kandy and stream flow (SF-cubic meters per second) at Peradeniya from 1944 to 1980 indicate in Fig 2. The average monthly rainfall reduced with 0.497 mm/month/year and average stream flow increase with rate of 0.15817 cubic meters/second/year. The reason may be rainfall at upstream locations (Nuwara Eliya) contribute for the stream flow at Peradeniya.

Data source : National Water Supply and Drainage Board (1994)

Figure 2: Variation of average monthly rainfall and stream flow

Correlation analysis

Correlation analysis done using 440 set of data using stream flow (cubic meters per second) of n^th month at Peradeniya (SFn), Ln (SFn), rainfall (in mm) of Kandy in n^th month (n-1)^th , (n-2)^th and (n-3)^th month are RFn, RFn-₁,  RFn-₂  and  RFn-₃ (in mm), respectively and average n^th month relative humidity during the daytime and night time at Katugastota are RHDn and RHNn respectively (as percentages).

Results of the correlation analysis using 441 observations for monthly stream flow indicated in Table 2. Among independent variables RFn, RFn-₁  and RHDn are high positive correlation with both SFn and Ln (SFn) dependent variables.  But  RFn-₃ and RHNn shows negative relation with both dependent variables. Based on the table 2, Ln (SFn) selected as dependent variable because Ln (SF) is highly correlated to dependent variables than SFn

Table 2: Correlation coefficients using 441 observations

	Ln (SFn)	SFn	RFn	RFn-1	RFn-2	RFn-3	RHDn
RFn	0.580483	0.608222	1
RFn-1	0.302218	0.221223	0.143114	1
RFn-2	0.036806	0.006208	-0.07108	0.14693	1
RFn-3	-0.22197	-0.19725	-0.2282	-0.07111	0.14481	1
RHDn	0.63528	0.497434	0.429387	0.355992	0.037462	-0.3569	1
RHNn	-0.1501	-0.1591	0.315644	0.091748	-0.18988	-0.1750	0.096508

The result of regression analysis is given in the Annexed Table 2. It shows the stream flow of Mahaweli Ganga during n^th month (SFn in 1000 cubic meters) at Peradeniya, rainfall at Kandy during the n^th month (RFn in mm), rainfall at Kandy in (n-1)^th , (n-2)^th and (n-3)^th month are RFn-₁,  RFn-₂  and  RFn-₃ respectively and average n^th month relative humidity during the daytime and night time at Kandy are RHDn and RHNn respectively (as percentages) by the following equation (N = 441, R = 0.801711, R² =0.642741).   The Standard Error is 0.47568 and following regression equation can use for stream flow calculation.  

The stream flow at Peradeniya with monthly average rainfalls day and night relative humidity at Kandy is given below for n^th month is given by using 441 observations. The related t statistic values are in the parenthesis.

Ln(SFn)  = 9.651741+ 0.003296RFn + 0.000808RFn-₁  – 0.00019RFn-₂ – (8.9E-05)RFn-₃ 

                             ( 8.105267)        (14.88073)        (3.906445)             (-0.95557 )           (-0.42377 )     

                               +   0.083092RHDn  – 0.13583RHNn         -----------------------------------------    (1)                   

                           (11.5492)                      (-11.7671)     

where SFn     - Peradeniya Stream flow n^th month in cubic meters per second

RFn     -  Kandy rainfall during n^th month in mm

RFn-₁  -  Kandy rainfall during (n-1)^th month in mm

RFn-₂ -  Kandy rainfall during (n-2)^th month in mm

RFn-₃ -  Kandy rainfall during (n-3)^th month in mm

RHDn   -Average daytime relative humidity Kandy during n^th month as %

RHNn  -Average nighttime relative humidity Kandy during n^th month as %

The equation (1) shows positive relationship for Ln(SFn) with n^th and (n-1)^th month rainfall values and relative humidity during day time (RHDn). Also equation shows negative relation with rainfalls during (n-2)^th and (n-3)^th months and relative humidity at night (RHNn). The relationship of n^th and (n-1)^th month rainfall values as well as RHDn can accept because rainfall directly relate with stream flow and when RHDn value high then water evaporation reduced and stream flow increases. 

Relationship between Predicted Ln (SFn) and Ln(SFn)

Relationship between the predicted Ln (SFn) and Ln (SFn) values are depicts in the Fig 3. According to this most points are close to Y =X   line and the relationship can be accepted for SF calculation.

Figure 3: Relationship between the predicted Ln (SFn) and Ln (SFn) values

Regression analysis for stream flow during seasons

Based on the regression analysis using each season monthly data from 1944 to 1980 the results are given below.

Relationship for stream flow at Peradeniya for southwest monsoon (SWM)period with monthly average rainfalls and day and night relative humidity at Kandy is given by equation 2 (N = 185, R = 0.726653, R² = 0.528025).   The Standard Error is 0.439649and following regression equation can use for stream flow calculation for SWM period.   Regression Statistics and t statistics values are given in Annexed Table 2. The equation shows all RF values shows positive relationship with Ln (SFn). Those relationships are accepted because four months rain water contribute for stream flow.  The equation shows both relative humidity values shows negative relationship with Ln (SFn).

Ln (SFn)  = 15.51081+ 0.003832RFn + 0.00101RFn-₁ +0.001099RFn-₂ +0.001432RFn-₃ 

                               -0.04726RHDn   -0.09988RHNn              …………………………….. (2)

Relationship for stream flow at Peradeniya for northeast monsoon (NEM) period with monthly average rainfalls and day and night relative humidity at Kandy is given by Equation 3. Stream flow at Peradeniya given by the following equation (N = 109, R = 0.827257, R² = 0.684355).   The Standard Error is 0.393318 and following regression equation can use for stream flow calculation for NEM period.  The equation shows all RF values shows positive relationship with Ln (SFn). Those relationships are accepted because four months rain water contribute for stream flow of nth month.  The equation shows both relative humidity values shows positive relationship with Ln (SFn). Those relationships are accepted because when relative humidity values are high then water evaporation reduced and stream flow increases. 

 Ln (SFn)  = -1.29689+ 0.00286RFn + 0.001428RFn-₁ +0.000373RFn-₂ -0.00013RFn-₃  +   0.020848RHDn  + 0.029701RHNn               ……………………….. (3)

Relationship for stream flow at Peradeniya for first inter monsoon (FIM) period with monthly average rainfalls and day and night relative humidity at Kandy is given by Equation 4. This analysis based on 73 observations with multiple R value of 0.786979, R square value of 0.619335and the standers error is 0.3743. The equation shows all RF values shows positive relationship with Ln (SFn). Those relationships are accepted because four months rain water contribute for stream flow of nth month.  The equation shows relative humidity values during day time shows positive relationship with Ln (SFn). This relationship is accepted because when relative humidity values are high then water evaporation reduced and stream flow increases. 

Ln(SFn)  = 1.78661+ 0.003841RFn + 0.002442RFn-₁ +0.000384RFn-₂ + 0.000621RFn-₃  +   0.02683RHDn   -0.01411RHNn               …………………………………….. (4)

Relationship for stream flow at Peradeniya for second inter-monsoon (SIM) period with monthly average rainfalls and day and night relative humidity at Kandy is given by Equation 5. This analysis based on 74 observations with multiple R value of 0. 0.631999, R square value of 0.399422 and the standers error is 0.3747. This equation  shows positive relationship for Ln(SFn) with n^th and (n-1)^th month rainfall values and relative humidity during day time (RHDn). These relationships can accept because rainfall directly relate with stream flow and when RHDn value high then water evaporation reduced and stream flow increases. 

Ln (SFn)  = 11.80845+ 0.001532RFn + 0.000246RFn-₁ - 0.00031RFn-₂ –0.00189RFn-₃  +   0.003297RHDn   -0.08364RHNn  …………………………………….. (5)

In all five equations shows positive relationship for Ln(SFn) with n^th and (n-1)^th month rainfall values and relative humidity during day time (RHDn). The relationship of n^th and (n-1)^th month rainfall values as well as RHDn can accept because rainfall directly relate with stream flow and when RHDn value high then water evaporation reduced and stream flow increases.  Negative relationships of other independent variables may be due to variation of other climatic parameters with seasons and soil conditions.

Water issues for each system

From 1995 to year 2000 water issues for Yala and Maha season for system B,C,G and H depicts in Figure 4.  Main feature of this figure is more amount of water issues for system H during Maha season than Yala season. Similar situation at system G  from 1996 to 1998. At all other systems Yala water issues are grater than that of Maha. Water issues for systems varies from 50.2MCN to 611MCM at System G during 1997 Yala and System H during 1995 Maha respectively.

Data source: Mahaweli Authority of Sri Lanka (2000)

Figure 4: Water issues for each system

Water duty for each system

Water duty means amount of irrigation water required (Hectare meters) for paddy cultivate in 1 Ha area. Water duty for each system indicated in Fig.5 for Yala and Maha. Except 1995 in System H all other systems and years duty for Yala is greater than that of Maha. The highest duty 3.39 m at System C during 1999 Yala season and the lowest duty 1.22m at System H during 2000 Maha season.

Data source : Mahaweli Authority of Sri Lanka (2000)

Figure 5: Water duty for each system

Descriptive statistics of water duty

Descriptive statistics of water duty shows by Annexed Table 3. According to that the highest value of mean duty (2.254) indicated in System B and the lowest value of mean duty (1.89) at System H. The second lowest  mean duty and the lowest standers deviation at System G. That mean water if we consider water savings then priority should be given to system H and G. 

Cultivated land area

Variation of cultivated area within Mahaweli System B, C,  G and H indicated in Figure 6. Cultivated land area of the System H is grater than all others and the fluctuation is high in the system H all tears except 1996 to 1998. This is agreed with above mentioned water duty statement. After 1994 cultivated area of all other systems show small increase. But cultivable area in System G is limited hence it remain at low value till year 2007.

Data source : Mahaweli Authority of Sri Lanka (2000)

Figure 6: Cultivated land area within Mahaweli System B, C, G and H

Water productivity

Water productivity means Kg of paddy production by 1 cubic meter of irrigated water. Variation of water productivity at Mahawely Systems depicts in Figure 7. Water productivity during the Maha season at System H shows increasing trend and higher value than all other systems except 1995. Water productivity during the Yala season at System G and H shows higher value than most of other systems. Water productivity during the Yala season at System H. G shows increasing trend from 1996. As indicated in Fig 6 more lands cultivated in System H is more productive and able to get good paddy harvest.

Data source : Mahaweli Authority of Sri Lanka (2000)

Figure 7: Variation of water productivity at Mahawely Systems

Descriptive statistics water productivity shows in Annex Table 4. According to that   the highest value of mean water productivity is at System H and the second highest at System C. System H shows the highest water productivity value (0.39 Kg/m³) and the lowest value (0.11 Kg/m³) at System B. Considering paddy production within Mahaweli using minimum amount of water then priority should be given to System H and C. According to Figure 6 cultivated land area in year 2007 within Mahaweli System H came 1^st System C, came 2^nd . That situation is favourable for paddy production concern.

Paddy production

Paddy production of the country and within the Mahaweli Project area indicated in the Fig 8. It shows the national paddy production varies within 2061,500 (in 1996) to 2859,900 Mt (in 2005) range but Mahaweli paddy production shows continuous increase from 256,438 (in 1984) to 707,840 Mt (in 2005). That shows under Mahaweli major irrigation paddy production shows increasing trend. Trend of national and Mahaweli paddy production within this period shows 26,692 and 18,536 Mt/year increasing trend respectively.  It shows good sign and even national production reduced in year 1992, 1996 and 97 Mahaweli production shows comparative low reduction to cover national requirement.

Figure 8: Variation of Paddy production of the country and within the Mahaweli Project area

Figure 9 depicts productions of paddy and other food crop (OFCs) cultivation at the System B, C, G and H of Mahaweli areas.  This shows increasing trend of OFC, banana and vegetable production from 1997 to 2005. This is helpful for the development of Mahaweli farmers and the country’s economy.

Data source : Mahaweli Authority of Sri Lanka (2000)

Figure 9: Variation of OFC production of the country and within the Mahaweli Project area

Conclusions and Recommendations

As the average annual rainfall at all stations in the Mahaweli Systemd reduced from 1931-60 period to 1961-90 and average monthly rainfall at Katugastota reduced with 0.497 mm/month/year.

The stream flow of Mahaweli Ganga at Peradeniya n^th month (SFn in 1000 cubic meters), can estimated using rainfall at Kandy during the n^th, (n-1)^th , (n-2)^th and (n-3)^th month are (RFn in mm), RFn-₁,  RFn-₂  and  RFn-₃ respectively and average n^th month relative humidity during the daytime and night time at Kandy are RHDn and RHNn respectively the stream flow is given by

Ln(SFn)  = 9.651741+ 0.003296RFn + 0.000808RFn-₁  – 0.00019RFn-₂ – (8.9E-05)RFn-₃ 

                                +   0.083092RHDn  – 0.13583RHNn  

The above relationship and equations (2) to (5) can be taken into account when planning for the increasing future water demand. As much as possible our water capacities of the reservoirs.

The highest water duty was 3.39 m at System C during 1999 Yala season and the lowest duty 1.22m at System H during 2000 Maha season. The attention should made to minimize water duty by researching and examining all water losses. 

If we able to maintain the highest water productivity value 0.39 Kg/m³  at all Mahaweli areas then with same water can produce double paddy production within Mahawely Systems. That is required for development of Sri Lanka. The excess water able to use for OFC cultivation and that will help to minimize food imports to the country. This is helpful for the development of Mahaweli farmers and the country’s economy.

All five regression equations should be tested using recent data set.  At present equations shows rainwater reach to stream flow after two months period. If we able to develop infiltration of rain water by developing forest cover etc we able to store rain water about three months. That is better than store in large reservoirs because of the danger of collapsing dams due to various natural and human activities.

The information on related studies should be collected and need to use for appropriate land use and other practices for optimize water resources for food production. Early study (Giragama 2005) indicates a positive correlation of runoff with increasing of home gardens as well as grassland areas. On the other hand increase of forest cover and tea land contribute to reduce the run off. Increasing extent of paddy lands also reduce run off according to the findings.

To maintain high water productivity and low water duty all canals and structures should repair and maintain properly for well functioning. The value of water resources and reuse it for other purposes should be promoted. Environmental policies should be strengthen for conservation of upper watersheds and all other voluble resources.

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Annexed

Annexed Figure 1: Mahaweli Development Project Area

Annexed Table 1: Characteristics of the newly created major reservoirs of the Mahaweli Ganga Development Scheme

Reservoir	Catchment km2	Area ha	Storage 106m3	FSL m	Impounded year
Maduru Oya	433	6390	473	96	1984
Victoria	1870	1300	728	438	1985
Kotmale Oya	544	520	175	703	1985
Randenigala	2333	2350	860	232	1986
Rantembe	3118		21	152
Ulhitiya/Rathkinda	280	2020	76/22	106	1984

Annexed Table 2: Regression Statistics and t statistics values.

Equation No.	1	2	3	4	5
Regression Statistics	Annual	SWM	NEM	FIM	SIM
Multiple R	0.801711	0.726653	0.827257	0.786979	0.631999
R Square	0.642741	0.528025	0.684355	0.619335	0.399422
Adjusted R Square	0.637802	0.512115	0.665787	0.584729	0.345639
Standard Error	0.47568	0.439649	0.393318	0.3743	0.374762
Observations	441	185	109	73	74
F	130.134	33.18973	36.85792	17.89682	7.426542
Significance F	9.85E-94	1.11E-26	1.89E-231	3.4E-12	4.07E-06
t Stat
Intercept	8.105267	4.505375	-0.33485	0.162711	1.625788
RFn	14.88073	11.3496	8.310942	5.650339	3.50766
RFn-1	3.906445	2.909181	4.509129	3.55918	0.632196
RFn-2	-0.95557	2.831649	1.218806	0.679034	-0.75928
RFn-3	-0.42377	3.922581	-0.36847	1.660363	-4.06558
RHd	11.5492	-1.49014	1.318675	0.725697	0.080906
RHn	-11.7671	-4.80168	0.5897	-0.09878	-0.90614

Annexed Table 3 Descriptive statistics of water duty

	System B	System C	System G	System H
Mean	2.254167	2.123333	1.984167	1.890833
Standard Error	0.133425	0.167157	0.097836	0.117888
Median	2.345	1.955	1.89	1.925
Standard Deviation	0.462197	0.579048	0.338914	0.408377
Range	1.53	1.96	1.05	1.37
Minimum	1.48	1.43	1.47	1.22
Maximum	3.01	3.39	2.52	2.59
Confidence Level(95.0%)	0.293666	0.36791	0.215336	0.259471

Annex Table 4:  Descriptive statistics water productivity

	System B	System C	System G	System H
Mean	0.1958333	0.23	0.2225	0.2375
Standard Error	0.0162116	0.01393	0.0125605	0.023936
Median	0.185	0.225	0.24	0.235
Standard Deviation	0.0561586	0.04824	0.0435107	0.082916
Sample Variance	0.0031538	0.00233	0.0018932	0.006875
Kurtosis	0.8526798	-1.8397	-1.093746	-0.76025
Skewness	0.9852078	0.01749	-0.369988	0.427215
Range	0.2	0.13	0.13	0.26
Minimum	0.11	0.16	0.15	0.13
Maximum	0.31	0.29	0.28	0.39
Sum	2.35	2.76	2.67	2.85
Count	12	12	12	12
Confidence Level(95.0%)	0.0356815	0.03065	0.0276454	0.052682