Flood vulnerability assessment in the light of rice cultivation characteristics in Mekong River flood plain in Cambodia

More than 2 million hectares of the lower Mekong River flood plain (Cambodian Plain), which extends from southern Cambodia to the border with Vietnam, is cultivated with rice; and more than 60 % of the population in the flood plain are farmers involved in rice cultivation. Rice production in this area is seriously affected by floods almost every year; on the other hand, the farmers need floodwater for their crop fields. Therefore, the farming in this area is about coping with floods, and flood countermeasures should be carefully planned based on good understanding of the livelihood of the farmers. Thus, this study aimed to identify flood-vulnerable rice-growing communities in the Mekong River flood plain in Cambodia. In this research, we proposed a useful methodology to make flood inundation maps by conducting a simple analysis by combining satellite-based digital elevation model (DEM) and river water level data based on the flood characteristics in this area. Then, rice-crop vulnerability maps were derived from previous maps and results from other past researches using the geographical information system (GIS). Comparison among those maps was also conducted to find out the relationship between DEM and people’s lifestyles in this area. This simple, inexpensive methodology was proven useful to understand major crop damage and vulnerability in relation to floods in this area, based on flood characteristics in the Cambodian flood plain. Because the method is a GIS-based approach, it can deliver more accurate results when provided with more accurate data.

     

Optimal gate operation of a main drainage canal in a flat,low-lying,agricultural area using a tank model

Chiyoda basin is located in Saga Prefecture in Kyushu Island, Japan, and lies next to the tidal compartment of the Chikugo River to which the excess water in the basin is drained away. Chiyoda basin has a total area of about 1,100 ha and is a typical flat and low-lying paddy-cultivated area. The main problem in this basin is the appropriate operation of drainage structures during and after flood events in order to minimize the inundation damages for crop yield and to fulfill the irrigation requirement in the irrigation period. This paper presents a mathematical model of a drainage system in Chiyoda basin for calculating the flood inundation and optimizing the operation of gates in a main drainage canal. First, the algorithm of gate operation was simulated and the drainage model was then evaluated by comparing the simulated water levels with those observed during an actual rainfall event. The results show that the observed and simulated water levels are in good agreement, indicating that the proposed model is applicable for drainage and inundation analyses in flat, low-lying paddy-cultivated areas. Second, the optimization of gate operation was investigated by trial and error method using a stochastic rainfall time series with a return period of 30 years and the tidal conditions of spring and neap tides in the Chikugo River. Comparing a total inundation time, a total inundation area and maximum inundation depth in the paddy tanks at the upstream and the downstream ends, it was concluded that the present operation based on the gate operators’ experience was almost the optimal one, and the sooner start of opening operation and the later start of closing operation within the operationality of check gates were recommended to minimize the inundation damage. The present operation could be able to minimize the total inundation time, the total inundation area and the maximum inundation depths in paddy tanks and to meet fully both the drainage and the irrigation requirements.     

Preliminary study of flow regimes and stream water residence times in multi-scale forested watersheds of central Cambodia

To elucidate the water cycle in lowland forests of the Mekong River basin, our research group established four experimental watersheds in the Stung Chinit River basin in Kampong Thom Province, Cambodia. The drainage areas of these experimental watersheds ranged from small (4 km²) to mesoscale (3,659 km²). Here, we present the first preliminary results of our rainfall-discharge observations and analyses of temporal variations of stable isotope ratios in rainfall, stream water, and groundwater. This paper focuses on the following three main topics: annual rainfall, discharge, and water balance; stormflow generation and dominant flow pathways; and flow regimes and stream water residence times. All stream water residence times (τ = 1.7–7.5 months) for the four experimental watersheds were shorter than the residence time of the groundwater (τ = 9.4 months) through the soil and regolith layers, implying that the stream waters consisted of not only the groundwater-flow component, but also younger-aged flow components such as saturation-excess overland flow. The smallest (4 km²; O Toek Loork) watershed had longer residence time (τ = 7.5 months) than the three larger watersheds (126–3,659 km²; τ = 1.7–3.9 months). This may suggest differing contributions of the groundwater and younger-aged flow components in the stream water in each watershed. Our approach of multi-scale watershed observation might better contribute to the needs of physically based models and aid in predictions for ungauged basins.     

Effective use of a reservoir for paddy irrigation in tropical monsoon Asia—a case study of the Ngamoeyeik Project,Lower Myanmar

Irrigation projects with reservoirs have contributed to the stabilization of traditional rainfed rice and to the introduction of double-rice in monsoon Asia. This type of irrigation also has the capability of shifting the traditional rice-cropping season. This paper discusses the influences of the shifted cropping season on water resources and the effective use of the reservoir in the Ngamoeyeik Irrigation Project Area in Lower Myanmar. Synthesized streamflow data were applied to simulate water storage in the reservoir. The main results are as follows: (1) shifting the cropping season to avoid inundation problems can increase water availability, (2) the amount of evaporation loss in the reservoir water budget can be as high as 16%, which decreases water efficiency in the reservoir operation during the dry season, (3) a shifted cropping season is preferable under the special hydrological conditions found in Lower Myanmar.     

Impacts of climate change on rice market and production capacity in the Lower Mekong Basin

The supply of water is affected by climate changes. In addition, high economic growth in the lower Mekong River countries including two large rice exporters is expected to reduce the per capita consumption of rice as incomes grow. Consequently, the need exist to investigate changes in supply and demand using econometric models of rice markets in the countries, where water supplies are expected to change. The objective of this research is to clarify impacts of water supply changes on rice producers and consumers using supply and demand models of rice considering changes in the water supply to aid in producing agricultural policies and plans. The following two simulations were conducted in this assessment: (a) baseline and (b) CC_B2. The simulation of the baseline includes the assumption that the evapotranspiration (ET) of each province after 2000 is the average quantity of ET from 1995 to 1999. The simulation of the CC_B2 includes the assumptions that ET of each province after 2010 accords with the IPCC socioeconomic scenario B2. The simulation results suggest that climate change will depress wet season rice production in Cambodia and Mekong River Delta (MRD) region in Vietnam, and that of dry season rice in the MRD region in Vietnam and northeastern region (Isan) Thailand. The simulation results also indicate that climate change will increase farm prices of rice in Cambodia, Vietnam, and Thailand. The results suggest that climate changes will weigh on the livelihood of rice consumers, especially those of poor rural populations.

     

Assessment of irrigation efficiencies and water productivity in paddy fields in the lower Mekong River Basin

Improving irrigation performance is a crucial issue for agriculture and irrigation development in the Lower Mekong River Basin to secure food production for people’s livelihoods. Irrigation efficiency is the most important indicator to determine the performance of an irrigation scheme. This study looks at water management practices and irrigation efficiency in three pilot sites in the Lower Mekong River Basin: the Numhoum scheme in Laos, the Huay Luang scheme in Thailand, and the Komping Pouy scheme in Cambodia. Irrigation efficiency and water productivity were analyzed using a water balance approach at the irrigation scheme level and results in the pilot areas show efficiencies that are definitely higher using this approach than by using the classical concept. Lower water productivity was observed at pilot schemes in areas of single cropping and higher productivity in areas where multiple agricultural activities were practiced. Strict and active water management is required to control and save water to meet agricultural demand and have sufficient water to expand cultivation areas while avoiding shortages. Promoting multiple uses of water for various agricultural activities in command area will increase water productivity.

Impacts of climate change on rice market and production capacity in the Lower Mekong Basin

The supply of water is affected by climate changes. In addition, high economic growth in the lower Mekong River countries including two large rice exporters is expected to reduce the per capita consumption of rice as incomes grow. Consequently, the need exist to investigate changes in supply and demand using econometric models of rice markets in the countries, where water supplies are expected to change. The objective of this research is to clarify impacts of water supply changes on rice producers and consumers using supply and demand models of rice considering changes in the water supply to aid in producing agricultural policies and plans. The following two simulations were conducted in this assessment: (a) baseline and (b) CC_B2. The simulation of the baseline includes the assumption that the evapotranspiration (ET) of each province after 2000 is the average quantity of ET from 1995 to 1999. The simulation of the CC_B2 includes the assumptions that ET of each province after 2010 accords with the IPCC socioeconomic scenario B2. The simulation results suggest that climate change will depress wet season rice production in Cambodia and Mekong River Delta (MRD) region in Vietnam, and that of dry season rice in the MRD region in Vietnam and northeastern region (Isan) Thailand. The simulation results also indicate that climate change will increase farm prices of rice in Cambodia, Vietnam, and Thailand. The results suggest that climate changes will weigh on the livelihood of rice consumers, especially those of poor rural populations.     

Factors impacting yields in rain-fed paddies of the lower Mekong River Basin

The Mekong River Basin (MRB) is the biggest basin in Monsoon Asia. About 80% of the agricultural lands, which occupy about 40% of the basin are rain-fed paddy rice area. Therefore, it is assumed that changes in rain-fed paddy rice production affect the total agricultural production to a great degree in the Mekong River Basin. While there are many factors affecting the productivity of rain-fed paddies, such as climate, water use, rice varieties, applications of manure, fertilizer and agro-chemicals, sowing date and other agronomic practices, this paper focuses on the relation between rainfall and yields of rain-fed paddies. Agricultural statistics and rainfall data were collected and analyzed for all 24 provinces in Cambodia for the years 2001 and 2002. Factors such as soil fertility and other natural conditions were removed by comparing the yield and rainfall in one province for different years. Special attention was given to the relation between yields of paddy in the wet season and rainfall, considering factors such as rice varieties, soil fertility, irrigation ratio and the ratio of area damaged by flood, drought, and insect. Although it is not easy to assess those impact factors on yields because they are organically interactive, the following results were obtained: (1) The ratio of high yielding varieties (HYV), soil fertility, and irrigation ratio among many factors that affect yields individually, especially if they are combined, (2) Total rainfall did not have a significant influence on rice yields even for the rain-fed paddies if it was over 700 mm in wet season, and one of the reasons for this would be that there exist supplementary water uses through small ponds and water ponding in local land depressions in and around paddies.     

A modeling approach for assessing rice cropping cycle affected by flooding,salinity intrusion,and monsoon rains in the Mekong Delta,Vietnam

We developed a crop scheduling model for rice cultivation in the Vietnam Mekong Delta (VMD), focusing on the adaptive behavior of crop planning to various water resource constraints. In addition, we also examined the effects of environmental change on rice cultivation in the last decade. In the VMD, multiple rice cropping is practiced under a variety of adverse water conditions, including flooding, salinity intrusion, and irregular monsoon rains. These environmental changes influence the durations of growing seasons and the number of crops per year, resulting in changes in productivity. To validate the performance of the model, we compared model estimates for the heading date and changes in leaf area index at nine sites with estimates of these parameters derived from MODIS satellite time series data for the period 2002–2006. The root mean square errors of heading date between the modeled and satellite data in the upper, middle, and coastal regions of the delta were 17.6, 11.2, and 13.0 days, respectively. Based on the model, we examined case studies to assess the changes in cropping cycles and crop failures in the VMD due to extreme flooding in 2000 and salinity intrusion in 2004 by applying evaluation indices defined by available period for cultivation (APC) and safe margin for cropping (SMC) which is defined as the marginal time between APC and the period required for cultivation. Findings of case studies suggested that a small difference in the SMC of the cropping pattern is critical to the stability and productive capacity of the rice crop.     

Estimation of supplementary water to paddy fields in the lower Mekong River basin during the dry season

Efficient management of water resources in paddy fields requires an understanding of the volume of supplementary water used. However, quantifying the volume is laborious due to the large amount of data that must be collected and analyzed. The purpose of our study was to estimate the volume of supplementary water used in paddy fields, based on several years of available statistical data, and to provide information on how much water can be supplied to paddy fields in each target area. In this study, the lower Mekong River basin of northeast Thailand, Laos, and Cambodia was selected as the study area. In the first step, we used agricultural statistics for each country, rainfall data acquired from the Mekong River Commission Secretariat (MRCS), and the value of virtual water required per unit of rice production. Because several years of data were used for dry season harvested areas and rice production in each country, the supplementary water to paddy fields in each province was calculated using virtual water and rainfall. This method made it possible to estimate changes in supplementary water in each province. Through this study, the supplementary water to paddy fields during the dry season in three countries was approximated from the minimum number of data sets. Moreover, for cases in which it is not possible to procure agricultural water use data for a hydrological model simulation, an alternative solution is proposed.     

Estimation of regional renewable water resources under the impact of climate change

The purpose of this paper is to develop an estimation model for regional renewable water resources under climate change conditions. Working with Japan??s high-resolution GCM-type JMA/MRI TL959L60 future rainfall modeling data in SRES-A1B condition, first we revise the modeling data through the Quadrant Transformation Method (QTM), and then use the Thornthwaite method for potential evapotranspiration to establish regional evapotranspiration data and estimate future renewable water resources. This study focuses on Taiwan??s four water resource regions to estimate renewable water resources for each region between 2080 and 2099. The results indicate that average annual renewable water resource volume for Taiwan as a whole will drop by 12.3% from 67.5 to 59.2?km³, as compared with the average from 1949 through 2000.     

A grid-based rainfall-runoff model for flood simulation including paddy fields

A grid-based, KIneMatic wave STOrm Runoff Model (KIMSTORM) is described. The model adopts the single flow-path algorithm and routes the water balance during the storm period. Manning’s roughness coefficient adjustment function of the paddy cell was applied to simulate the flood mitigation effect of the paddy fields for the grid-based, distributed rainfall-runoff modeling. The model was tested in 2296 km² dam watershed in South Korea using six typhoon storm events occurring between 2000 and 2007 with 500 m spatial resolution, and the results were tested through the automatic model evaluation functions in the model. The average values of the Nash–Sutcliffe model efficiency (ME), the volume conservation index (VCI), the relative error of peak runoff rate (EQ_p), and the absolute error of peak runoff (ET_p) were 0.974, 1.016, 0.019, and 0.45 h for calibrated storm events and 0.975, 0.951, 0.029, and 0.50 h for verified storm events, respectively. In the simulation of the flood mitigation effect of the paddy fields, the average values of the percentage changes for peak runoff, total runoff volume, and time to peak runoff were only −1.95, −0.93, and 0.19%, respectively.     

Survey of Rice Cropping Systems in Kampong Chhnang Province,Cambodia

Although Cambodia might have achieved self-sufficiency and an exported surplus in rice production,its rice-based farming systems are widely associated with low productivity,low farmer income and rural poverty.The study is based on a questionnaire village survey in 14 communes containing 97 villages of Kampong Chhnang Province from March to June,2011.It analyzes the prevailing rice-based cropping systems and evaluates options for their improvement.Differences in cropping systems depend on the distance from the Tonle Sap water bodies.At distances greater than 10 km,transplanted wet-season rice cropping system with low productivity of about 1.6 t/hm 2 prevails.This deficiency can be primarily attributed to soils with high coarse sand fractions and low pH (< 4.0),use of ’late’ cultivars,and exclusive use of self-propagated seeds.To improve this cropping system,commercial ’medium’ cultivars help prevent crop failure by shortening the cultivation period by one month and complementation of wet-season rice with non-rice crops should be expanded.Areas adjacent (≤ 1 km) to the water bodies become inundated for up to seven months between July until January of each year.In this area,soils contain more fine sand,silt and clay,and their pH is higher (> 4.0).Farmers predominantly cultivate dry-season recession rice between January and April.Seventy-nine percent of the area is sown directly and harvested by combines.Adoption ratio of commercial rice seeds is 59% and yields average 3.2 t/hm 2.Introduction of the second dry-season rice between April and July may double annual yields in this rice cropping system.Besides upgrading other cultivation technologies,using seeds from commercial sources will improve yield and rice quality.Along with rice,farmers grow non-rice crops at different intensities ranging from single annual crops to intensive sequences at low yields.     

Spatio-temporal variations in meteorology drought over the Mekong River Delta of Vietnam in the recent decades

Paddy and Water Environment - In recent years, Mekong Delta of Vietnam is severely affected by salinity intrusion and water scarcity due to climate variability. In this study, a comprehensive...     

Impact of seawater intrusion control on the environment,land use and household incomes in a coastal area

Agricultural production in the coastal wetlands of Asia is often hindered by salinity intrusion caused by tidal fluctuation. This paper reports changes in environmental and socio-economic conditions that followed the phased construction and operation of sluices for controlling seawater intrusion from 1994 –2000 in a coastal area of the Mekong River Delta, Vietnam. Canal water salinity decreased rapidly upstream of sluices, allowing rice cropping intensification and increased rice production in the eastern part of the study area. However, the livelihoods of farmers in the western part were adversely affected due to cessation of supply of brackish water that was needed for brackish-water shrimp farming, while the acid sulphate soils present there posed problems for rice cultivation. The poor farmers and landless people suffered more because the fishery resource that they depended on declined sharply due to reduced salinity and increased acidity in the canal water. The findings confirmed that the environment and resource use in the coastal lands are very sensitive to external intervention. A clear understanding of the socio-economic and environmental impacts of salinity control measures in coastal areas can help planning to enhance farmers' incomes while minimizing negative environmental impacts. Land-use policy formulation, planning and management should adopt a more holistic approach, taking into account the interests of all resource users, especially the poor, instead of focusing on any particular sector.     

A water balance model for characterization of length of growing period and water stress development for rainfed lowland rice

There is large year-to-year variation in rice production across the Mekong region (Laos, Cambodia and Thailand) due to uncertainty in the timing of the onset of the wet season and drought stress that may develop at any time during the growth of rainfed lowland rice. Unique to the nature of lowland water balance is a large component of deep percolation water loss, which depends on soil texture. The objectives of this study were to develop a soil water balance model for calculating the amount of water held in field storage (i.e. in soil and, if there is standing water, above the soil surface) and to apply it to determine the length of growing period (LGP) and water stress development in relation to soil type and rainfall pattern for the rice ecosystem. The water balance is computed separately for above-ground plus topsoil layer and subsoil layer. Components of the water balance are the existing amount of stored water, rainfall, evapotranspiration, deep percolation, and runoff. The deep percolation rate was determined from clay content in each soil layer. The model runs with daily or weekly weather data to estimate the soil water level for the growing period in the wet season. The model was validated with data collected from top, middle and bottom of rainfed lowland fields in Savannakhet province, Laos. The best correlation between the observed and simulated water level was obtained (r² = 0.41) for middle fields. The simulation results showed that LGP varied greatly from year to year, particularly in locations with sandy soils, due mostly to variation in monthly rainfall occurring at the early part of the growing season (April), but also to some extent by variation at the end of growing season (October). Soil texture on the other hand is shown to have a large influence on the end of the rice growing period and hence LGP, and also water stress development during growth. Sandy soils with clay content less than 7% that are prevalent in the province are shown to cause frequent water stress and early finish in rainfed lowland rice. The model accordingly provides reasonable outputs that can provide a geographical dimension of soil hydrological patterns for various rice growing environments, and also identify the spatial pattern of drought stress that is likely to occur. Model outputs can be used to provide guidelines for practical advice to the rice farmers and researchers for determination of appropriate crop management strategies (e.g. time of planting, varieties), and policy makers for investment decisions on inputs (e.g. fertilizer price) aimed at increasing rice productivity in this Mekong region.     

Operation of the Ba Lai irrigation system in the Mekong Delta, Vietnam

Ba Lai Irrigation Project is located at coastal area in the Mekong Delta. In dry season when the flow rate decreases and the strong east wind blows into the delta, the salinity intrusion increases and seriously affects agricultural and domestic water use. Intakes of Ba Lai system have to be closed for 1–3 months depending on their locations, and no water supply during this period often causes water pollution in the project area. In order to solve such problems, this study aims to seek gate operation procedures for salinity control and water environment improvement. A numerical model is developed to simulate water movement, salinity concentration and duration of remaining water (water age) within the system under three scenarios: (1) without control structures, (2) with available control structures, (3) with the full control structures. Through the numerical simulations, control structures are confirmed to be an effective measure for the salinity control and suitable gate operation schedules are proposed to improve the water environment in the project.     

Evaluation of land suitability for Moringa oleifera tree cultivation by using Geospatial technology: The case of Dhidhessa Catchment,Abay Basin,Ethiopia

Land suitability analysis of Moringa oleifera tree cultivation is important to enhance its product, as the demand for this tree for medicinal values and food sources is increasing worldwide. Therefore, this study aimed to assess suitable land for Moringa oleifera tree cultivation by using the integration of multi-criteria evaluation with geospatial technologies in the Dhidhessa catchment, western Ethiopia. Five parameters, namely: slope, land use and land cover(LULC), soil texture, land surface ...     

Features of the AFFRC model for evaluating the relationship between the water cycle and rice production

This paper introduces the Agriculture, Forestry and Fisheries Research Council of Japan (AFFRC) model, an integrated model that predicts future rice production in the Mekong River basin by taking into account the effect of global warming on both the water cycle and the rice economy. The model focuses especially on the water balance of paddy fields for different farmland water use systems. We defined six categories of irrigated paddies and three categories of rain-fed paddies on the basis of their systems of water usage. We included a process-based model to predict future rice production, accounting for daily changes in available water resources such as precipitation. Many models of crop production treat rice in the same way as other crops; the particular characteristics of rice farming are considered in more detail in our model. Our results show that it is possible to estimate future rice production in the Mekong River basin by taking into account changes in available water, and to model the resultant effects on the grain market.     

Evaluation of MODIS NDVI and LST for indicating soil moisture of forest areas based on SWAT modeling

This study examined the capability of remotely sensed information gained using the terra moderate resolution imaging spectroradiometer (MODIS) normalized difference vegetation index (NDVI) and land surface temperature (LST) to explain forest soil moisture. The soil and water assessment tool (SWAT) was used for the analysis. Nine years (2000–2008) of monthly MODIS NDVI and LST data from a 2,694.4 km² watershed consisting of forest-dominant areas in South Korea were compared with SWAT simulated soil moisture. Before the analysis, the SWAT model was calibrated and verified using 9 years of daily streamflow at three gauging stations and 6 years (2003–2008) of daily measured soil moisture at three locations within the watershed. The average Nash–Sutcliffe model efficiency during the streamflow calibration and validation was 0.72 and 0.70, respectively. The SWAT soil moisture showed a higher correlation with MODIS LST during the forest leaf growing period (March–June) and with MODIS NDVI during the leaf falling period (September–December). Low correlation was observed in the year of frequent rains, regardless of the leaf periods.