changes over time. Based on the WRSI for individual crops, LEAP permits the calculation of a basket WRSI reflecting the WRSI for a basket of crops grown in a given area.

- (iii) **_Calculation of yield reduction:_** As an intermediary output, the LEAP tool computes yield reduction estimates. A 0% yield reduction would reflect that the harvest would be at its potential, and a yield reduction of 100% would reflect a complete crop failure. Yield reduction is computed for each crop and each region, and is dependent on actual evapotranspiration, total water requirement without water stress and a yield response factor representing the effect of a reduction in evapotranspiration on yield losses<sup>43</sup> . Based on yield reduction estimates for individual crops, LEAP permits the calculation of yield reduction of a basket of crops grown in a given area.

- (iv) **_Calculation of estimated beneficiary numbers:_** Based on previously computed WRSI data in combination with past beneficiary numbers and demographic data, the LEAP tool computes estimated beneficiary numbers. In particular, the LEAP tool uses a logarithmic model to compute needs based on the following parameters:

- a. **_Needs in case of optimal rainfall_ :** Number of people in need of assistance in case of optimal rainfall in any given region. In particular, the level of needs in case of optimal rainfall may be interpreted as the number of chronically food insecure people, as their status of food insecurity is independent of crop performance.

- b. **_Population at risk of food insecurity_ :** This number corresponds to the maximum number of beneficiaries in the case of the most severe drought possible, and should include the entire population at risk of food insecurity due to drought in a given area. To optimally estimate this parameter, this variable should be based on field assessments to assess the number of people faced with drought-related food insecurity. However, in the absence of recently updated data, LEAP selects the maximum historical beneficiary number recorded in a given area.

- c. **_Lowest observed value of RWRSI:_** The Regional Water Requirements Satisfaction Index (RWRSI) is defined as the weighted average of woreda-level combined WRSI, where the weights are given by the population for each woreda. The lowest observed RWRSI reflects the lowest drought index based on historical data, reflecting the most severe drought on record in a given area. In cases where a drought occurs that is more severe than the most severe past drought, the model used is able to extrapolate the associated level of needs.

- d. **_Optimal RWRSI:_** The optimal RWRSI refers to the optimal rainfall conditions relative to a given region. Although a value of WRSI of 100% would reflect optimal crop growth conditions, the optimal RWRSI may be less than 100% depending on local conditions.

- e. **_Systemic failure level:_** This parameter estimates the complete failure of the crop seasons, reflecting the level of water stress beyond which community coping mechanisms are exhausted and beneficiaries require external assistance to meet food needs. The parameter is difficult to estimate, as it depends on farming practices and farmers’ skills. However, the failure level should be selected such that it is significantly lower than the lowest observed value of RWRSI (see c. above). One possibility for selecting the systemic failure level is to use a value of WRSI of 50, below which crops are assumed to fail, or to link the systemic failure level to the lowest observed value of RWRSI by scaling down the lowest observed value of RWRSI through a constant assumed to be the threshold beyond which crops fail.

> 43 Doorenbos and Kassam (1979).

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