Study on water stress distribution of rice production in terraced paddy fields : case study of Wonogiri central Java Indonesia

概要

Food security is often threatened by droughts during rice production. Although most of the rice is produced in lowland paddy fields, terraced paddy fields are also important in the rice production system in the island or mountainous countries. Terraced paddy serve as the main income source for the locals and has many other functions for the ecosystem. However these area is seemingly to face the risk of abandonment due to migration of younger generation and rice production damage due to climate uncertainty. The intensifying frequency of El Niño periods in recent decades increasing the risk on rice production. Though the potential impacts of such events on rice production have been examined variably from regional to the national scale, the impact on the local area especially in intensively cultivated terraced paddy area is yet to be reported. In the area where ground monitoring data is scares such as mountain area, remote sensing could be an efficient and reliable tool in providing measures for drought mitigation.

The main objective of this study was to evaluate drought condition in terraced paddy area. The study was conducted using remote sensing analysis coupled with hydrological monitoring data. Remote sensing analysis was conducted using two indices, CWSI (Crop water stress index) and TVDI (Temperature vegetation dryness index), derived from MODIS (Moderate-resolution imaging spectroradiometer) for 2015 to 2018 period. Keduang watershed was chosen as the study area as it has distinct feature where around 30% of the land use is terraced paddy area. It is located in the southern part of Mount Kendeng in Wonogiri Regency, Central Java, Indonesia.

The remote sensing analysis showed the apparent changes of drought condition in the study area. The results indicated that droughts normally start to be detected in the beginning of dry season and reached the peak in between September to November. Furthermore in the El Niño year drought was intensified and the impacted area was also expanded while in La-Niña comparably most area remained in the normal condition. The spatial distribution of drought condition was also presented in the study area. Irrigated and upstream area tend to show lower drought intensity compare to the more downstream area in the same river system. Rainfed area also had higher drought intensity compared to the irrigated one. From the rice production data, it could be inferred that the temporal and spatial distribution showed by drought indices was in line with the tendency of rice production condition. However, the change on drought condition did not affect rice production significantly.

Hydrological monitoring was conducted to support the detail condition of hydrology condition in the terraced area. Ponding water depth and groundwater table in the observed field tend to be stable in the normal year but began to decrease in the prolonged dry period in 2018. The depression of groundwater table in the upper field of terraced paddy concomitant with the decrease of ponding water depth while the groundwater in the middle and lower fields remained stable. Though the result showed that dry condition could increase the risk for rice cultivation security, as in ponding water depth, in the top fields of terraced paddy, rice production did not show significant difference of the three observation plots. Monitoring in the river stream druing 2015 – 2016 also showed that terraced paddy contribute in the groundwater recharging and groundwater flow then contribute on the river flow for irrigation in terraced paddy area.

Generally, TVDI and CWSI were visible to monitor drought in terraced paddies, but they are still not accurate enough because the complexity of terraced paddies is different from normal land conditions. Nevertheless, through this research we found that rice cultivation in terraced paddies was relatively stable under the different climate conditions. Additionally, groundwater contributes considerably to stabilizing the hydrology conditions in terraced paddy areas. Further study of the modification of the drought indices need to be conducted to obtain more reliable drought analysis of terraced paddy areas toward damage mitigation of drought in the event of intensifying ENSO.