Using the system dynamics approach for utilization of wastewater in agriculture sustainability (Case study: Varamin plain)

Agriculture in Varamin plain has been faced with many challenges, due to vicinity to metropolitan
Tehran the capital of Iran (competition for Latian dam reservoir), and competition with Tehran south
network in allocation of Mamlou dam reservoir and treated wastewater of south wastewater treatment
plant. Based on national policy, the treated wastewater should be replaced with Latian dam reservoir
water to supply water demand of agricultural sector. Several factors affect on wastewater management
and success of utilization plans and any change in these factors may have various feedbacks on the
other elements of wastewater use system. Hence, development of a model with capability of simulation
of all factors, aspects and interactions that affect wastewater utilization is very necessary. The main
objective of present study was to model and simulate long-term effects of irrigation with Tehran treated
wastewater on water and soil resources using System Dynamics (SD) modeling approach. The
evaluation of developed SD model (VarSim) showed that the model has capability to simulate key
variables such as crop yield, groundwater levels (RMSE=0.37m and ME=0.45m) and ground water
salinity (RMSE=57 μS/cm and ME=148μS/cm) with high accuracy. The most important scenarios
evaluated consisted of four wastewater allocation scenarios [(i) keeping the excising condition, (ii)
complete allocation of Latian dam reservoir water to Tehran domestic use, allocation of 200 MCM
treated wastewater during growing season to agricultural sector and 40 MCM to artificial aquifer
recharge during non-growing season starting year 1395, (iii) similar to scenario number two w/o
artificial aquifer recharge and (iv) similar to scenario number two plus allocating 50 MCM starting year
1410], four crop pattern scenarios, three deficit irrigation scenarios, six pressurized irrigation systems
scenarios and six water and energy pricing scenarios. Model results showed that the keeping the
current condition would lead to complete degradation of groundwater resources in the study area by
2042. In the meantime, results showed that in the case of exercising of the second scenario of
wastewater allocation, ground water resource degradation can be postponed till 2067. In the case of
exercising of fourth scenario or with second scenario along with a 10% increase in irrigation efficiency
through deficit irrigation or modern irrigation systems development can result in sustainable water
resources use in Varamin plain. In this study, for evaluation of the effectiveness of the wastewater
scenarios, two national indices resulted from groundwater quality degradation and water level decrease
and national net benefit index were defined. The result showed that with increasing wastewater supply
of southern wastewater treatment plant and also using artificial aquifer recharge, compensation costs of
aquifer quality degradation and groundwater level decrease is reduced and national income is
increased. Total compensation costs in wastewater allocation scenarios of number one, two, three and
four are, 18320, 107786, 16512 and 8545 billion Rials, respectively, in the period of 35 years. In
summary, fourth and first scenario had the most income and the lowest income, respectively. Among
the four scenarios of crop pattern, the existing crop pattern had the highest income, the lowest national
cost and highest water productivity. Results also indicated that unplanned development of pressurized
irrigation systems not only didn’t save the water resources, but it resulted in damaging groundwater
resources. While using surface water resources, if cultivated area is not increased, then development of
sprinkler systems leads to increase national income and groundwater level. Otherwise the increase of
cultivated area results in reduction of gross national income. Development of pressurized irrigation
systems leads to improvement of groundwater quality and quantity level with and w/o cultivated area
increment. Thus, the development of pressurized irrigation systems can help to sustainable
development only if cultivated area remains constant and water consumption is reduced.

Using the system dynamics approach for utilization of wastewater in agriculture sustainability (Case study: Varamin plain) Keywords:

Treated wastewater , National income , System dynamic , Agriculture sustainability and Varamin plain