This week I’ll summarize two very relevant articles on #drought concepts that I found during the current literature review I’m doing for my MSc. thesis . These papers are key to my research and they where written by Ashok K. Mishra and Vijay P. Singh (2010) in the Journal of Hydrology.
The purpose this summary is to give brief overview of what a drought actually is and how it has been studied over the years, and for what purposes.
Drought as natural hazard.
A natural hazard is a threat of a naturally occurring event that will have a negative effect on people or the environment. The reasons for the occurrence of droughts are quite complex, since they depend not only in the atmosphere but also on the hydrologic processes, which feed moisture to the atmosphere. The lesser the relative humidity the less probable the rainfall becomes, as it will be harder to reach saturation conditions.
Droughts rank first among all natural hazards when measured in terms of number of people affected (Obasi, 1994; Hewitt 1997). For this reason it is important to understand the nature of droughts.
- The onset and the end of a drought are very difficult to determine, the impacts of a drought increase slowly and often accumulate over a period of time
- It is difficult to define a drought because there is no universal definition of drought.
- Drought impacts are non-structural and spread over large geographical areas.
- Human activities can directly trigger a drought unlike other natural hazards.
Classification of droughts.
- Meteorological drought: Lack of precipitation over a region for a period of time.
- Hydrological drought: A period of inadequate surface and subsurface water resources for established uses of a given water resource management system.
- Agricultural drought: Declining soil moisture and consequent crop failure.
- Socio-economic drought: When the demand of water as an economic good exceeds supply as a result of shortage in water supply.
- Groundwater drought: When groundwater systems are affected by droughts, first ground water recharge and later groundwater levels and discharge decrease.
How to measure/detect drought?
Drought indices are prime variables for assessing the effect of a drought and defining parameters such as: intensity, duration, severity and spatial extent. All of this indices have their strengths and limitations which I will not describe in this post.
Standardized Precipitation Index (SPI): Long-term precipitation records are fitted to a probability distribution, then it is transformed to a normal distribution and analyzed.
Palmer Drought Severity Index (PDSI): Precipitation and temperatue are analyzed for estimating moisture supply and demand within a two layer soil model.
Crop Moisture Index (CMI): Using weekly values of temperature and precipitation to compute a simple moisture budget. Used to evaluate short-term moisture conditions (week to week).
Surface Water Supply Index (SWSI): Hydrological drought index calculated on monthly non-exceedance probability form historical records of reservoir storage, streamflow, snow pack and precipitation.
Vegetation Condition Index (VCI): Satellite remote sensing provides a synoptic view of the land and spatial context for measuring drought impacts which have proved to be a valuable source of timely, spatially continuous data on monitoring vegetation dynamics over large areas.
What are scientists looking for?
Drought forecasting: There are several statistical models: regression, time series, drought probably and hybrid models.
Probabilistic characterization on parameters such as: severity, duration, intensity frequency area.
Spatio-temporal analysis: the impacts of droughts are evaluated with the parameters previously mentioned through the area of study and through its evolution in time.
Modeling climate change scenarios: Derived from global climatic models (GCM) spatio-temporal climate variability and its effects on the hydrology are modeled.
Land assimilation system for drought models: Remote sensing satellite observations on: land cover classification and a priori parameterization of vegetation dynamics.
To overcome the challenges due to increased drought recurrences and to reduce impacts there needs to be strategies to maintain water security.
A) Impacts of natural triggered hydro-meteorological variability on water resources.
B) Planning using the information obtained from models above mentioned.
Decision Support Systems (DSS)