System dynamics based model for conjunctive management of water: Jaffna Peninsula in Sri Lanka. K.D.W. Nandalal University of Peradeniya Sri Lanka

Transcription

1 System dynamics based model for conjunctive management of water: Jaffna Peninsula in Sri Lanka K.D.W. Nandalal University of Peradeniya Sri Lanka GWSP international conference 'Sustainability in the Water-Energy-Food Nexus'2014

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3 Rainy season in Jaffna districts is during the northeast monsoon from October to December Annual rainfall Rainfall during this period is about 70% of the annual total rainfall Average monthly rainfall

4 Area is relatively flat. Surface drainage under normal rainfall is meager Heavy runoff and flooding however takes place after heavy rains in the rainy period through the drainage courses More than 50% of northeast monsoonal rainfall is received within a short period It results in large losses due to surface runoff

5 Jaffna Peninsula is mainly dependent on groundwater for all its water requirements

6 A model was developed for Jaffna Peninsula to identify water availability in different phases and water usage for diverse purposes The peninsula was modeled at Divisional Secretariat Division (DSD) levels using system dynamics programming technique Jaffna Peninsula is divided into fifteen DSDs in the model. They are called water units in the model.

7 Since the availability of most of the data is at DSD level, this division was used in the model Each unit comprises of three sectors, viz., water quantity, population and agriculture All fifteen units, each with the above three sectors, are integrated to build the overall model for the whole peninsula The model has been developed using the Vensim (Ventana Systems Inc., 2004) modeling environment

8 Basic model structure of different sectors including the main variables

9 Water sector in each water unit (DSD) is independently developed and hydraulically connected via surface and subsurface inflow and outflow as appropriate Water sector has three basic components; water inflow, water use and water outflow Inflows to a unit are precipitation and surface and subsurface flows from neighbouring water units Precipitation either recharges groundwater zone or flows overland to streams to become streamflow Water use in a unit consists of agricultural, industrial, domestic, commercial water requirements and water losses due to evapotranspiration

10 Agricultural sector is developed at water unit level Crops considered are paddy and highland crops Birth, death and net migration are the main issues of the population sector Population sector is also modelled at the water unit level and fertility and mortality are considered at this level The population is directly related to the drinking water requirements

11 Model component for Thenmarachchi DSD Model was calibrated using data from 1995 to 1997 It was verified for the period 1998 to 1999

12 Water system within the peninsula was simulated over the period from 2000 to 2012 Average rainfall over the Jaffna peninsula on average is observed to be about 1460 MCM/yr Drinking water demand is about 19 MCM/yr, while agricultural water use is 228 MCM/yr Average amount of water infiltrating through the ground surface and percolating further downward is observed to be about 770 MCM/yr

13 It is assumed that the evapotranspiration from the soil above the water-table is approximately 50 percent of this volume, which is amounting to 385 MCM/yr. Out of the surface runoff of about 690 MCM/year, about 345 MCM/year is used to satisfy evapotranspiration demand while about 225 MCM/year is discharged into the sea. The balance flows to the three lagoons The remainder contributes to net recharge to the aquifer system, which is about 26% of the average annual rainfall

14 The total evapotranspiration, 895 MCM/yr, is about 61% of the average annual rainfall received over the peninsula Amount of water entering Vadamarachchi lagoon is about 72 MCM/yr and the amount entering Upparu lagoon is about 39 MCM/yr while about 8 MCM/yr enters Elephant Pass lagoon

15 Change in groundwater storage in the Valikamam area The variation of groundwater level is observed to be upto about 3 m during the simulation period of 13 years

16 Groundwater level variation within a year in the Valikamam East DSD Average groundwater use in the Valikamam East DSD

17 Surface water available per unit area at DSD level Groundwater use at DSD level

18 Surface water available per unit area at DSD level

19 Some Conclusions Model is capable in identifying water in different phases Groundwater level is not affected by the present use There are 992 small ponds in the district and they are in a poor condition. Cleaning them will improve percolation of rain water and will recharge the aquifers. Practicing effective water management measures, crop diversifications, micro irrigation methods (sprinklers, drip irrigation), etc., are necessary to conserve groundwater. Extend the model to have economy added as a sector

20 Thank you