F. Todisco

Analysis of agricultural drought characteristics through a two-dimensional copula

The potential of a copula model for the description of the joint probability distribution of two agricultural drought characteristics, the relative onset RO and the relative severity RS, is investigated in Perugia (Central Italy) in reference to a sunflower crop. The 1924–2009 time series of daily precipitation and maximum and minimum temperature were used to simulate, by means of the AquaCrop model, the root-zone soil water dynamics (SWt) and the crop yield under rainfed conditions. The seasonal values of RO and RS were quantified, by applying the theory of runs to SWt (assumed as the drought reference variable), with a threshold equal to the crop critical point. The analysis shows that the best-fitting marginal distribution for both RO and RS is a truncated Gumbel distribution. The dependence structure of RO and RS, investigated by graphical and analytical techniques, was modeled by a Student copula, which is able to adequately reproduce both the overall and upper tail dependence among variables. Lastly, the Student copula was applied to obtain joint probabilities and bivariate return periods for RO and RS. These results, compared with the expected estimated yields, provide useful information for drought planning and management. For example, for the case study considered, it was found that the condition RO ≥ 0.43 (i.e., onset before the end of June) and RS ≥ 0.22 has a 5-year return period and is frequently associated with critical yields, and that the condition RO ≥ 0.47 (i.e., onset before mid-June) and RS ≥ 0.25 has a 10-year return period and is almost certainly associated with critical yields.

New Methodology for the Risk Analysis and Economic Impact Assessment of Agricultural Droughts

The paper proposes a new methodology that studies agricultural drought not only in terms of deficit soil water content, its frequency and severity, as in the traditional approach, but also in terms of net benefit. For this purpose the relationship between drought index, crop yield, and crop economic net benefit is modeled and three threshold levels are considered. The first is the critical water content, below which the crop faces water stress (termed as potential agricultural drought). The second is the critical net benefit, below which there is meager (e.g., zero) profit (termed as actual agricultural drought). The third is the critical return period (or frequency) of the actual agricultural drought, below which there is an agricultural aridity (almost steady phenomenon with high impact, high risk) and above which there is an agricultural drought (occasional phenomenon with high impact, low risk). To pinpoint the time of occurrence of the potential and the actual agricultural drought, real-time monitori...

Operative Approach to Agricultural Drought Risk Management

In a companion paper, the agricultural drought economic risk assessment (ADERA) model was described. ADERA studies agricultural drought not only in terms of deficit soil water content and its frequency and severity (as in the traditional approach) but also in terms of net benefit reduction and vulnerability. The relationship between drought index, crop yield, and crop economic net benefit is modeled and three threshold levels are considered: the critical water content; the critical impact in terms of net benefit; and its critical return period (i.e., frequency) above which the soil-climate unit is considered unsuitable for the cultivation of a specific crop. These critical levels are used in long-term action planning as the triggers for different risk classes. The rapidity with which the critical impact is reached is considered indicative of the crop vulnerability, and real-time monitoring of the impact evolution (short-term actions) is also implemented. Initially ADERA was applied at only one point (soil-crop-climate unit), but because agricultural drought is by definition a wide area phenomenon, the model must be validated in other units and then used to evaluate the drought area extension (risk map). In this paper, generalization, demonstration, and guidance on the application of ADERA are given with reference to rainfed sunflower and sorghum crops in six soil-climate units in central Italy for which time series for yield (obtained in experimental plots where the water scarcity was the principal factor limiting the final yield) since 1978 is available. The application of ADERA made it possible to quantify the vulnerability class and the corresponding risk class for the single unit (long-term action planning). The analysis results confirm that typically for the selected crops the critical impact has a low risk and a low vulnerability. In fact sunflower and sorghum are rainfed crops widely cultivated in the area under investigation. Moreover, the results show that each risk class is present only within a specific cost range, in particular the upper and lower limits, and the size of each cost range varies according to the different units; the occurrence of a specific risk class and vulnerability level is highly correlated with the selected triggers, in particular with the critical net benefit (for an increment of which above the corresponding cost range, a given soil-climate unit can become unsuitable for the cultivation of a specific crop); the critical water content is a normal occurrence event for all the units analyzed, with the crop water stress onset date always around 40% of the growing period being a typical characteristic of the specific area, of the two crops and of the selected drought index. The practical usefulness of the impact monitoring procedure was stressed by comparing its progressive evolution in the cases of both a rainfed crop and of a rescue irrigation carried out at a selected and opportune date. In the latter case the impact was reduced below the critical level.