3. Method for agricultural damage assessment

### 3.1 Framework of flood damage assessment

Flood risk and damage assessment are essential for flood risk management. The main purpose of flood damage assessment is to identify areas at risk where mitigation actions are necessary. This chapter describes a grid-based method for flood damage assessment considering three major factors of risk: hazard, exposure, and vulnerability. Flood damage assessment starts with the identification of the target area and items exposed to a flood hazard using the results of flood hazard simulation and moves on to the evaluation of damage that might occur to exposed items because of the vulnerability of each item. Figure 3 shows the estimation process of agricultural flood damage assessment. Flood damage can be assessed by combining knowledge of a flood hazard and an item exposed to the flood with vulnerability. Flood characteristics such as flood depth and duration can be simulated using a

of disasters in the basin. The magnitude of damage due to the target flood is an important factor in identifying the target scale for flood management and can be simulated in the process of risk assessment. To identify the magnitude of a flood of a specific return period, rainfall data should be used for statistical analysis as the primary information of the target natural hazard. Discharge volume is also applicable to statistical analysis if no overflows from the river occur in the upstream of the measurement point and the land use of the catchment area has not changed. The water level of the river is not appropriate for statistical analysis because the river cross section at the water-level observation point often changes in an alluvial floodplain. As mentioned above, rainfall data is normally employed as a statistical sample for a study on the design flood scale. To assess a flood hazard of a specific return period, a rainfall hyetograph for a specific return period can be estimated by multiplying the selected rainfall pattern of a past flood event by a conversion factor, as shown in Figure 4. The conversion factor for each return period can be calculated as the ratio of the corresponding rainfall of the return period and the rainfall volume of the selected rainfall pattern. Normally, the rainfall pattern with the highest rainfall volume is selected among the past flood events for designing a

Schematic figure for designing a rainfall hyetograph for a specific return period flood.

Methodology for Agricultural Flood Damage Assessment DOI: http://dx.doi.org/10.5772/intechopen.81011

After assessing hazard areas by a hydrological/hydraulic simulation, paddy areas exposed to the flood can be identified by overlaying the flood hazard areas and the paddy areas to assess flood damage to rice crops. The paddy-field areas can be extracted from a land-cover map prepared by using satellite images or land-use and land-cover maps prepared by a local government. Several freely available global

For the case studies, paddy fields were extracted using a land-cover map prepared by NWRB and JICA [12] for the Pampanga River basin of the Philippines and a global land-cover map prepared by the Global Land Cover by National Mapping Organizations (GLCNMO) [13] for the Lower Indus River basin. The details will be

After the identification of exposed paddy fields in the hazard areas, possible damage can be calculated using risk indicators. Each risk indicator represents the

rainfall hyetograph for a specific return period.

3.3 Identification of exposed paddy fields

Figure 4.

land-cover data are presented in Table 1.

discussed in the section of case studies.

3.4 Agricultural damage assessment

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3.4.1 Depth-duration-damage function curves

Figure 3. Estimation process of rice-crop damage caused by flooding.

hydrological model, and paddy areas can be extracted from land-cover and land-use maps to identify exposed paddy areas in the hazard-risk areas. Then, the yield loss caused by flooding can be estimated by applying flood damage curves and converted to economic value based on the value of farm gate price and rice yield.

### 3.2 Flood hazard assessment

The identification of flood hazard areas and the intensity of a hazard by hazard assessment is the first step for flood risk and damage assessment. Flood hazard assessment is conducted by applying a hydrological/hydraulic simulation model using hydrometeorological data, topographic data, land-use data, and operation rules of river management structures. Information on past flood hazards, such as rainfall, river water level, discharge volume, and inundation area and depth, is required to develop and calibrate a simulation model.

For the case studies in the Pampanga and Lower Indus River basins, the rainfallrunoff-inundation (RRI) model, developed by Sayama et al. [10], was employed to compute flood characteristics such as inundation depth and duration. The RRI model is a two-dimensional model capable of simulating rainfall-runoff and flood inundation simultaneously [11]. The model deals with slopes and river channels separately. The flow on the slope grid cells is calculated with a 2D diffusive wave model, while the channel flow is calculated with a 1D diffusive wave model. The details of the RRI model can be found in Sayama et al. [10] and Sayama [11]. The model parameters were calibrated to the past largest flood event cases. Flood hazard assessment was conducted for the past largest flood event and 100-year flood cases in the Pampanga River basin and for the past largest flood event case in the Lower Indus River basin. For the simulation of the target flood of a specific return period, statistical analysis was conducted to identify the magnitude of a hazard of the target scale.

Generally flood risk assessment is conducted for a probable future flood event of a specific return period or for the past largest flood event. The target scale differs by river according to the socioeconomic activities, expected flood damage, and history Methodology for Agricultural Flood Damage Assessment DOI: http://dx.doi.org/10.5772/intechopen.81011

Figure 4.

hydrological model, and paddy areas can be extracted from land-cover and land-use maps to identify exposed paddy areas in the hazard-risk areas. Then, the yield loss

The identification of flood hazard areas and the intensity of a hazard by hazard assessment is the first step for flood risk and damage assessment. Flood hazard assessment is conducted by applying a hydrological/hydraulic simulation model using hydrometeorological data, topographic data, land-use data, and operation rules of river management structures. Information on past flood hazards, such as rainfall, river water level, discharge volume, and inundation area and depth, is

For the case studies in the Pampanga and Lower Indus River basins, the rainfallrunoff-inundation (RRI) model, developed by Sayama et al. [10], was employed to compute flood characteristics such as inundation depth and duration. The RRI model is a two-dimensional model capable of simulating rainfall-runoff and flood inundation simultaneously [11]. The model deals with slopes and river channels separately. The flow on the slope grid cells is calculated with a 2D diffusive wave model, while the channel flow is calculated with a 1D diffusive wave model. The details of the RRI model can be found in Sayama et al. [10] and Sayama [11]. The model parameters were calibrated to the past largest flood event cases. Flood hazard assessment was conducted for the past largest flood event and 100-year flood cases in the Pampanga River basin and for the past largest flood event case in the Lower Indus River basin. For the simulation of the target flood of a specific return period, statistical analysis

Generally flood risk assessment is conducted for a probable future flood event of a specific return period or for the past largest flood event. The target scale differs by river according to the socioeconomic activities, expected flood damage, and history

was conducted to identify the magnitude of a hazard of the target scale.

caused by flooding can be estimated by applying flood damage curves and converted to economic value based on the value of farm gate price and rice yield.

3.2 Flood hazard assessment

Estimation process of rice-crop damage caused by flooding.

Recent Advances in Flood Risk Management

Figure 3.

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required to develop and calibrate a simulation model.

Schematic figure for designing a rainfall hyetograph for a specific return period flood.

of disasters in the basin. The magnitude of damage due to the target flood is an important factor in identifying the target scale for flood management and can be simulated in the process of risk assessment. To identify the magnitude of a flood of a specific return period, rainfall data should be used for statistical analysis as the primary information of the target natural hazard. Discharge volume is also applicable to statistical analysis if no overflows from the river occur in the upstream of the measurement point and the land use of the catchment area has not changed. The water level of the river is not appropriate for statistical analysis because the river cross section at the water-level observation point often changes in an alluvial floodplain.

As mentioned above, rainfall data is normally employed as a statistical sample for a study on the design flood scale. To assess a flood hazard of a specific return period, a rainfall hyetograph for a specific return period can be estimated by multiplying the selected rainfall pattern of a past flood event by a conversion factor, as shown in Figure 4. The conversion factor for each return period can be calculated as the ratio of the corresponding rainfall of the return period and the rainfall volume of the selected rainfall pattern. Normally, the rainfall pattern with the highest rainfall volume is selected among the past flood events for designing a rainfall hyetograph for a specific return period.

#### 3.3 Identification of exposed paddy fields

After assessing hazard areas by a hydrological/hydraulic simulation, paddy areas exposed to the flood can be identified by overlaying the flood hazard areas and the paddy areas to assess flood damage to rice crops. The paddy-field areas can be extracted from a land-cover map prepared by using satellite images or land-use and land-cover maps prepared by a local government. Several freely available global land-cover data are presented in Table 1.

For the case studies, paddy fields were extracted using a land-cover map prepared by NWRB and JICA [12] for the Pampanga River basin of the Philippines and a global land-cover map prepared by the Global Land Cover by National Mapping Organizations (GLCNMO) [13] for the Lower Indus River basin. The details will be discussed in the section of case studies.

#### 3.4 Agricultural damage assessment

#### 3.4.1 Depth-duration-damage function curves

After the identification of exposed paddy fields in the hazard areas, possible damage can be calculated using risk indicators. Each risk indicator represents the


#### Table 1.

List of globally available land-cover data.

vulnerability of each item by showing the correlation of the intensity of a hazard with damage quantified by a damage curve. Therefore, flood damage curves are important in flood damage estimation. To estimate flood damage to rice crops, depth-duration-damage function curves are normally used.

Flood damage curves can be mainly derived from two approaches: (1) using actual damage data of past floods and (2) using synthetic data (expert estimation or questionnaire surveys) [8]. In the former approach, flood damage curves are developed based on data and information of past hazards and resulting actual flood damage. Therefore, accumulation of data on hazards (inundation records) and flood damage is essential. In the latter approach, damage curves are derived from hypothetical analysis, information obtained from questionnaire surveys, or land cover and standardized typical property types.

Table 2 shows the flood damage matrix for rice-crop damage published by the Philippines Bureau of Agricultural Statistics [14]. Figure 5 shows the height of rice plants at each growth stage and its duration. Based on the flood damage matrix and the information on rice plant height at each growth stage, Shrestha et al. [16] proposed flood damage curves for rice crops as presented in Figure 6. Flood damage curves of rice crops vary with each rice growing stage. Based on the duration of each growth stage of rice plants and the information on a cropping calendar, the growth stage of rice plants during a flood event can be identified, and an appropriate damage curve corresponding to the growth stage of rice plants should be applied


for damage estimation. For the case studies in this chapter, the flood damage curves for rice crops presented in Figure 6 were used to estimate agricultural economical

Flood damage to rice crops is defined as a function of flood depth, flood duration, and growth stage of rice plants and can be estimated by using the calculation

losses.

117

Figure 6.

Figure 5.

Height of rice plants at each growth stage and its duration [14, 15].

Methodology for Agricultural Flood Damage Assessment DOI: http://dx.doi.org/10.5772/intechopen.81011

Depth-duration-damage function curves for rice-crop damage [16].

3.4.2 Damage estimation method

Table 2.

Flood damage matrix for rice crop published by the Philippines Bureau of Agricultural Statistics [14].

Figure 5. Height of rice plants at each growth stage and its duration [14, 15].

Figure 6. Depth-duration-damage function curves for rice-crop damage [16].

for damage estimation. For the case studies in this chapter, the flood damage curves for rice crops presented in Figure 6 were used to estimate agricultural economical losses.
