Drought forecasting based on seasonal climate forecasting in combination with hydrological and crop growth models

1. a historical reconstruction of the terrestrial water cycle running over many decades to produce the soil moisture and precipitation climatology, which facilitates the comparison of current and predicted conditions;  
2. a real-time monitoring system to track present drought conditions; 
3. bias-corrected and downscaled seasonal forecasting data produces seasonal hydrological predictions and derives drought maps and other hydrological products up to six months in advance. 

Drought events are indicated when the results of the SPI for whichever timescale is being investigated become continuously negative and reach a value of −1. The drought event is considered to be ongoing until SPI reaches a value of 0. The ability of the SPI to be calculated at various timescales allows for several applications. Depending on the drought impact in question, SPI values for three months or less might be helpful for essential drought monitoring, values for six months or less for monitoring agricultural impacts and values for 12 months or longer for hydrological impacts.

SPEI uses the basis of the SPI but includes a temperature component, allowing the index to account for the effect of temperature on drought development through a basic water balance calculation. SPEI has an intensity scale in which both positive and negative values are calculated, identifying wet and dry events. It can be calculated for time periods between one month and 48 months. Monthly updates allow it to be used operationally and the longer the time series of data available, the more robust the results will be. With the same versatility as that of SPI, SPEI can be used to identify and monitor conditions associated with a variety of drought impacts.

This index is calculated using monthly temperature and precipitation data and information on the water-holding capacity of soils. It considers moisture received (precipitation) and moisture stored in the soil, accounting for the potential loss of moisture due to temperature influences. Although it was developed mainly to identify droughts affecting agriculture, it has also been used for identifying and monitoring droughts associated with other types of impacts as well.

This index uses daily precipitation data to develop and compute several parameters: effective precipitation (EP), daily mean EP, deviation of EP (DEP), and DEP's standardised value. These parameters can identify the onset and end of water deficit periods. Using the input parameters EDI calculations can be performed for any location in the world. The results are standardised for comparison, giving a clear definition of drought onset, end, and duration. This is a good index for operational monitoring of both meteorological and agricultural drought situations because calculations are updated daily.

This index, by calculating a basic soil water balance takes into account the impact of drought. It identifies when the drought stress occurred within the development of the crop and what the overall effect on the final yield will be. PDSI and CMI can identify drought conditions affecting a crop, but do not indicate the likely impact on yields

This is a weekly soil moisture product calculated at four different soil depths, including the total soil column, at 0.61, 1.23 and 1.83 m and can be used as an indicator of short-term drought, especially using the results from the 0.61 m layer.

This is a weekly product that helps identify water stress for crops. ETDI is calculated along with the Soil Moisture Deficit Index (SMDI), in which a water stress ratio is calculated that compares actual evapotranspiration with reference crop evapotranspiration. The water stress ratio is then compared with the median calculated over a long-term period. ETDI is very useful for identifying and monitoring short-term drought affecting agriculture.

The case study area is in the mid-Guadalquivir valley in Andalucía in the south of Spain. This region is well known for its agricultural productivity and its efforts for improving the efficiency of resources. These efforts included significant investment in modernising irrigation systems in most Water Users Associations of the territory. It is a traditional citrus-producing area where citrus orchards occupy more than 60% of the surface and the tendency is to increase the area of woody crops. Managers and farmers can benefit from early warning tools/services to detect water deficits in woody crops.