CENGKLIK RESERVOIR PERFORMANCE AND ITS ROLE FOR DROUGHT MITIGATION

Water availability problem is encountered by Cengklik Reservoir due to drought disaster in the current year. It causes irrigation water crisis over 850 hectares crop field which of 350 hectares were not cultivated. The risk that must be faced by farmers is decrease in potential productivity, losses about more than 2.5 billion. Therefore, it needs technical solution to reduce this drought disaster risk. To obtain an alternative solution against water availability problem for drought disaster mitigation, this research used optimization of reservoir standard operating simulation. It applies field area of rice or Palawija at the second and/or the third cultivation season as decision variable, maximum productivity value as objective function, irrigation water demand as parameter depending on specified alternative crop pattern and schedule, and several constraints comprising 100% of reservoir reliability, all field is irrigated at the first and second season in which maximum non-irrigated crop field at the third cultivation season are 300 hectares. The tool used to conduct optimization was Microsoft Excel software. The result showed that crop pattern considered as an alternative solution against water availability problem in Cengklik reservoir is paddy-paddy-maize at the early of November II, cultivated over 433 hectares and 1524 hectares. Risk reduction reached 9.33% in term of reservoir reliability, 23.61% in term of irrigated area, and 27.29% in term of vulnerability towards water availability crisis.


INTRODUCTION
Cengklik Reservoir as one of reservoirs used for food production control in Boyolali Regency has been dealt with water availability problem induced to drought disaster.Drought which has occurred for the last ten years hinders irrigating land area of 462 hectares located in downstream of division structure BCi 12 over 9 villages in 2 sub-districts.The Solopos Newspaper published on 4 April 2009 stated that 850 ha over 5 villages could not be irrigated.It caused about field area of 350 ha being neglected, while the rest which was cultivated used groundwater pumping system since no water supply from Cengklik Reservoir.The encountered risk is approximately 2.5 billion rupiahs of loss of potential productivity.
The aim of the research was to find out solution regarding to water availability problem by applying technical procedure of reservoir management so that entire Cengklik Irrigation area can be irrigated on Crop Field I and Crop Field II, and minimalize the non-irrigated area on Crop Field III.

STANDARD OPERATING RULE
Setiawan (2007) revealed that water release management on the multipurpose reservoir can be conducted by using standard operating rule, as seen in Figure 1.

R(t) = R T (2) if R T < S(t) + I(t) ±E(t) ± '6 " 5 T + Kw -DS R(t) = S(t) + I(t) ± E(t) ± Kw
(3) if S(t) + I(t) ± E(t) > R T + Kw R(t) = 0 (4) if S(t) + I(t) ±( W " '6 where R T is target volume of water release (m 3 ), R(t) is water release volume at t time (m 3 ), S(t) is reservoir storage at t time (m 3 ), I(t) is inflow volume at t time (m 3 ), E(t) is water losses due to evaporation on the reservoir at t time (m 3 ), DS is minimum storage of reservoir (m 3 ), and Kw is reservoir capacity (m 3 ).As depicted by Figure 1, it can be explained that basically, simulation is carried out by setting trial and error value of target release (R T ) such that optimum parameter in water utilization can be obtained.Simulation of reservoir storage can be calculated by following equation.
where t is total of discrete time (24 period in 15 days), S t+1 is reservoir storage at t time (m 3 ), S t is reservoir storage at the end of time (m 3 ), I t is inflow volume at t time (m 3 ), E t is water losses due to evaporation on reservoir at t time (m 3 ), O t is release/outflow volume at t time (m 3 ), and Kw is reservoir capacity (m 3 ).
In order to achieve optimum value, degree of reliability is determined as follows: where R is degree of reliability (%), n is number of failed reservoir operation within half-monthly period, N is data length in half month.
Optimization is a process of activity to gain the best result which is conducted repetitively and mutually influence.The best result is indicated by the minimum or maximum value.

STEP OF ANALYSIS
The used data was rainfall and climatology data, irrigation network scheme, crop pattern, crop implementation, water irrigation supply, measured release discharge, and storage volume of the reservoir.There were three steps of analysis comprising water availability analysis, water irrigation demand, and water balance analysis.Water irrigation demand analysis is based on evapotranspiration yielded from climatology data analysis, combined with crop pattern, schedule, and irrigation efficiency.From the result of water availability and water demand analysis, served irrigation area can be determined whether it has been in accordance with the existing design.If the result do not fit the requirement, crop pattern and schedule will be modified.Simulation was done many times until whole area on Crop Field I and Crop Field II was served, the non-irrigated area on Crop Field III decreased which was not more than 300 ha, and gaining the maximum production result.

Characteristic of Cengklik Reservoir
The reservoir characteristic that shows the relationship among the elevation, storage capacity and inundated area is presented in Figure 2.

Water Availability on Cengklik Reservoir
Water availability depends on the amount of inflow to the reservoir.Inflow volume is calculated based on water balance analysis.Mean inflow of half-monthly period is shown in Figure 3. Figure 3 described that crisis of water availability in Cengklik Reservoir has been occurred since 1994 indicated by declining mean inflow drastically from 0.75 m 3 /s in 1993 to 0.4 m 3 /s in 1994.In 1997, halfmonthly mean inflow was less than 0.4 m 3 /s, it caused about crop field area of 462 ha located at downstream of division structure Bci 12 was non-irrigated within 1998-1999.On the following years, half-monthly mean inflow has never reached 0.5 m 3 /s (equivalent to 15.5 MCM/year), whereas it used to be surpassed in 1997.

Water Irrigation Demand
Data which was used to determine potential evaporation (Eto) consists of climatology data including maximum and minimum temperature, relative humidity, wind acceleration, solar radiation, and astronomy data containing elevation and location of climatology station.The results of potential evaporation analyzed using Penman Monteith method was provided in Table 1.Percolation parameter in crop field area is based on Directorate General of Irrigation (1986) which is 2 mm/day.
Effective rainfall is defined as depth of rainfall which was consumed by plants to substitute water losses due to evapotranspiration, percolation, and so on.It was calculated using mean algebra method since rainfall station surrounding the observation area is well-distributed.Effective rainfall on half-monthly period is shown in Figure 4.
Variation of water demand in cultivating paddy field is shown in Figure 5. Water demand analysis for Palawija was somewhat similar with the analysis of paddy field, yet for Palawija, it used 15 days T, and 50 mm S. Water demand variation in Palawija case is shown in Figure 6.      6.Based on the water availability and crop realization data, k factor can be calculated which will be used for water irrigation supply analysis.Water irrigation is ratio between given discharge and total water demand within a period of time.Based on the interview, farmers applies intermittent water irrigation system with duration time of 2x24 hours/week.It gives k factor of 0.805.Water balance in Cengklik Reservoir with intermittent system, k is 0.805, and served area according to aforementioned Decision Letter is shown in Figure 7.
Figure 7 shows that if the existing crop pattern is implemented with the crop field area followed Decision Letter of the Regent, it may deal with drought disaster which causes failed in harvesting time.It is indicated by the amount of potential release which is less than target release.It can be found in 4 th year thru 10 th year on the above graph.Optimization was carried out by using Microsoft Excel software.To know the feasibility of alternative crop pattern, monthly rainfall and ten daily rainfall should be examined as shown in Table 7 and Table 8.Table 7 shows that the highest rainfall is 165 mm/month occurred on January, so that no wet month indicated in this irrigation area.From Table 8, it can be seen that the third week on January is the right time to conduct plantation in rain fed, yet productivity of paddy become very small because of low rainfall probability at ripening period.

Figure 4 .
Figure 4. Effective rainfall in Cengklik Irrigation Area for paddy and Palawija.

Figure 5 .
Figure 5. Water demand variation for land preparation of paddy.

Figure 6 .
Figure 6.Water demand variation for land preparation of Palawija.

Figure 7 .
Figure 7. Water balance in Cengklik Reservoir with intermittent system, k=0.805, served area = 1,957 ha per Crop Field.

Table 1 .
Potential evaporation L/s/haThe first 15 days land preparationThe second 15 days land preparation

Table 9 .
Recapitulation of the optimization results of crop pattern and schedule

Table 9 .
Recapitulation of the optimization results of crop pattern and schedule (continued)

Table 10
5HVHUYRLU RSHUDWLRQ VLPXODWHG EDVHG RQ 5HJHQW ¶V Agreement Letter No. 521/569 in 2008 using Paddy-Paddy-Paddy/Maize crop pattern yields irrelevant result with the current condition since about 975 -1200 ha area is non-irrigated for achieving 200% reliability at Crop Field III.c) Paddy-Paddy-Maize at Crop Field I on November II provides the most optimum result for drought disaster mitigation.Productivity value per year reaches Rp 74,879,516,800.00.d) Risk mitigation has been achieved through modification of crop pattern and schedule is approximately 9.33% in term of reservoir reliability, 23.61% in term of irrigated area, and 27.29% in term of vulnerability.5HJHQW ¶V $JUHHPHQW /HWWHU DERXW FURS SDWWHUQ DQG design is necessary to re-evaluate in order to make more adaptive to overcome the risk of drought disaster.b) Paddy-Paddy-Maize applied at Crop Field I, November II is considered to be alternative solution against the lack of water availability in