by Aswandi; (Thesis M.Sc Unesco-IHE, Delft, Netherland, 2016) supervised Robiyanto H. Susanto, Edward Saleh, and Iskhaq Iskandar
The development of drainage system over peat land areas causes the reduction of groundwater table. This condition increases oxidation process leading to peat decomposition, which in turn impacts on abundant release of C02. This environmental impact is an important part of global warming mitigation. Peat oxidation is also responsible for rapid land subsidence in the drained areas and thus decreases land productivity. Accurate simulation of the drainage and its impacts is one of urgent needs to provide optimal way of land management. Therefore, we need a couple of models that is capable on simulating various scenarios of water management and estimating the land subsidence and C02 emission rate. Simulations
from various scenarios can provide useful options to decide the best practice of water management by considering maximum profit of crops and minimum environmental damages (ratio of profit and loss). Two numerical models have been developed: GCFlow (Groundwater-Channel Flow) and EmSub (Emission and Subsidence). The former aims to simulate day-to-day variation of groundwater table controlled by canals and rainfall. The latter aims to simulate emission and subsidence by taking into account the annual average of water table simulated from GCFlow model. Both models run interchangeably to obtain 3-dimensional (x,y,t) results with a 10 m spatial resolution. GCFlow uses two components of sub-models: open-channel flow and groundwater flow. Integration from these components enables the analysis of drainage system. Accuracy of GCF low model has been verified for 2-years simulation of daily water table in a secondary block in Rantau Makmur Village, Berbak-Tanjabtim, Jambi. Correlation of determinatioan R2 can reach 0.958 with an average of 0.84, indicating reliability of GCFlow model. Several scenarios of drainages are simulated with GCF low model. Their annual-mean water tables are used as the input in EmSub model to calculate peat oxidation because of drainage scenarios. EmSub simulation shows that deep water table scenarios cause very large losses on CO2 emission and land subsidence. Strong subsidence is susceptible to inundation that is not suitable for agricultures. Because of fast inundation, maximum time of usable land becomes shorter than usual. EmSub model predicts the land condition in 100 years simulation. Using 0.8 m average water table (current condition), the land potentially releases 794.000 ton CO2 or losses Rp 61.2 billion, causes 52 cm average subsidence, and leaves only 623% land cover which is available for plantations. The losses become smaller or bigger if the drainage becomes shallower or deeper, respectively. Using 0.1 m average water table, the model simulates only 279.500 ton C02 emission (Rp 21.5 billion), 19.8 cm average subsidence, and 100% area of usable land cover. However, using 1.5 m water table, the model simulates 1.4 million ton CO2 emission (Rp 108 billion), 90.6 cm subsidence, and only 35.9% land cover. Changes in drainage levels have big impact on productivity and profit of crops and plantations because every crop/plant needs different drainage. Generally, simulations in a range of (a) age of plant and (b) 100 years simulation show that food crops group (e. g., paddy and corn) is the most profitable as shown by high profit/loss ratio compared with industrial plantations group (e.g., oil palm and acacia). The results also suggest that drainage system in oil palm plantation and acacia causes the largest damages because of deep water table, therefore their profit/loss ratio are very small. In case of food crops, paddy has the highest ratio. Other food crops (corn, peanuts, etc.) and some combinations also have high ratios because they do not need deep drainage. Consistent with the initial assumption, depth of water table in drainage is proportional to amount of damages: increasing CO2 emission, increasing subsidence which causes shorter age of land cover, and reduction of profit/loss ratio for any plants. In this context, we suggest a sustainable water management. The cause of this issue is due to improper management of water-peat-drainage.
Keywords: drainage, water, peat, C02 emission, subsidence, spatial