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The Standardized Precipitation Evapotranspiration Index is a multiscale drought index based on climate data. It is designed to take each precipitation and potential evapotranspiration (PET) in determining drought. Drought is the absence of water over an extended period of time. Parameters such as rainfall and temperature can be used as indicators of drought. PET is calculated using six empirical methods. They are Hargreaves, Turc, Makkink, Priestly-Taylor, Ritchie and Penman-Monteith.SPEI can be calculated on a range of timescales from 1-48 months.
In this study we calculated for 12-month time scale.
PET is a measureof the ability of the atmosphere to remove water from the surface through the process of evaporation and transpiration. Rate of evapotranspiration is affected by sunlight, surface humidity, temperature, solar radiation, and wind. Precipitation minus potential evapotranspiration gives climatic water balance. Given a time series of climatic water balance, it gives a time series of the SPEI. In this study, we calculate SPEI for Rajendra Nagar using six empirical methods and compare other five empirical methods with Penman-Monteith method.
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Drought classification and Moisture categories are based on the SPEI values. If SPEI >-1 then there is no drought. We found 10 drought years which includes moderate drought and severe drought in the given time period. In this study we found Turc, Ritchie and Makkink showed better results while comparing to standard Penman-Monteith method.
Drought is a natural hazard with a significant impact on the economic, agricultural and environmental. It is defined as a significant decrease in the availability of water in all its forms.
Rainfall deficits over a given period of time can lead to varying degrees of drought. It is one of the biggest threats to human survival. It can be distinguished in terms of intensity, location, duration and time. It can be considered a multilevel phenomenon guided by the response time of the basin. It should be noted that hydrological responses to soil moisture, river discharge, and groundwater discharge vary and have different response times.
Therefore, the time at which water deficits accumulate (time scale) is very important to determine the prevailing type of drought. The performance of drought indices varies by application and specific region. Out of all drought indices, SPEI is the most commonly used drought indices. The SPEI is based on the difference between precipitation (P) and reference evapotranspiration (ETo) accumulated in a certain period of time. Evapotranspiration is a loss of water to the atmosphere by the combined process of Evaporation (E) and Transpiration (T). The rate of evapotranspiration is mainly controlled by temperature, wind speed, and water availability. Temperature is one of the major climatic factors that affect water availability and also has effects on nature and drought. Potential evapotranspiration is a measure of the ability of the atmosphere to remove water from the surface through the processes of evaporation and transpiration.
This difference between (P) and (ETo), two crucial components in agricultural drought monitoring, represents irrigation requirements in arid and semi-arid regions.SPEI has a very good performance in monitoring summer drought. For the calculation of SPEI we used six empirical methods Hargreaves, Turc, Makkink, Priestley-Taylor, Ritchie and Penman-Monteith for estimating ETo.Penman-Monteith (PM) is recommended by the Food and Agricultural Organization of the United Nations (FAO). Evapotranspiration is calculated for Rajendra Nagar station for 50 years (1965-2015) on a daily basis. Later it will be converted to monthly data as we have monthly precipitation data. All other methods are compared with PM as it is considered as the standard method because it considers the main meteorological factors. However, the PM method has very high demands which often makes its calculation complicated when the data is unavailable or inaccurate. The accuracy of the SPEI largely depends on the selected (ETo) method. In this study we compare all five empirical methods with Penman-Monteith method for the calculation of SPEI.
The study area Rajendra Nagar is a Mandal located in Ranga Reddy district of the Indian state of Telangana with the elevation of 538m.the region refers to (17° 19' 8.832'' N, 78° 23' 31.0848'' E)
The data set consists of fifty years (1965 - 2015) of daily records of solar radiation (Rs) , Temperature (T), relative humidity (RH) and wind speed (U2).In the below Table 1, the Xmean, Sx, Cv, Csx, Xmin and Xmax denote mean, standard deviation, variation coefficient, skewness, minimum and maximum respectively.
Table 1: Statistical Parameters of Rajendra Nagar Dataset
| Variable | Mean (Xmean) | Std. Deviation (Sx) | Coefficient of Variation (Cv) | Skewness (Csx) | Min (Xmin) | Max (Xmax) | Correlation with ETo |
|---|---|---|---|---|---|---|---|
| Rs | 9.32 | 2.44 | 0.26 | -1.08 | 3.55 | 14.4 | -0.05 |
| Tm | 26.1 | 3.92 | 0.15 | -0.07 | 14.5 | 36.7 | 0.37 |
| RH | 60.7 | 14.9 | 0.25 | -0.16 | 10 | 139 | 0.55 |
| U2 | 4.69 | 4.62 | 0.99 | 0.85 | 0 | 142 | 0.26 |
| ETo | 2.89 | 1.48 | 0.51 | 0.28 | -1.49 | 22.5 | 1 |
From the above Table 1 Relative humidity is closely correlated with evapotranspiration (ETo) calculated using Penman-Monteith Method. The temperature is the second best parameter correlated with ETo.
The ET0 values for Rajendra Nagar station are calculated using the FAO-56 PM method and other five empirical methods Hargreaves, Turc, Makkink, Priestly-Taylor and Ritchie.
The Penman-Monteith method (PM) is generally considered as a standard method as it requires more meteorological data. It calculated on a daily basis. Where Rn is the net radiation at crop surface (MJ m-2 d-1),G is the soil heat flux (MJ m-2 d-1),T is the average temperature at 2 m height(°C),U2 is wind speed measured at 2 m height [m s-1],(es - ea) is pressure deficit for measurement at 2 m height [k Pa],D is slope vapor pressure curve [k pa°C-1],g is psychrometric constant [k pa°C-1],900 is coefficient for the reference crop [l J-1 Kg K d-1],0.34 is wind coefficient for the reference crop [s m-1]
The Hargreaves method estimates ET0 based on maximum and minimum air temperature, it is calculated on a daily basis. Where Td is the difference between maximum temperature and min temperature (°C), Tm is mean temperature (°C), Ra is extraterrestrial radiation (mm day-1)
The Turc method estimates ET0 based on maximum and minimum air temperature and solar radiation. It is calculated on a daily basis. Where Tm is mean temperature (°C), solar radiation (Rs) is [0.25 + 0.5 (n/N)] Ra, Ra is extraterrestrial radiation (mm day-1), n is actual hours of bright sunshine (hrs.), N is maximum possible hours of sunshine (hrs.)
The Makkink method was proposed by De Bruin (1987). It calculated using daily basis. Where D is slope vapor pressure curve [k pa°C-1], g is psychrometric constant [k pa°C-1], solar radiation (Rs) is [0.25 + 0.5 (n/N)] Ra, Ra is extraterrestrial radiation (mm day-1), n is actual hours of bright sunshine (hrs.), N is maximum possible hours of sunshine (hrs.)
The priestly method is calculated using Net radiation and latent heat of vaporization. It is calculated on daily basin. Where D is slope vapor pressure curve [k pa°C-1], g is psychrometric constant [k pa°C-1], Rn is the net radiation at crop surface (MJ m-2 d-1), A is a calibration constant 1.26, L is the latent heat of vaporization (MJ/kg)
The Ritchie method was proposed by Jones and Ritchie in 1990. It calculated using Air temperature and Solar radiation. It calculated on a daily basis. Where Tmax and Tmin are maximum and minimum temperatures, A1 is 1.1, solar radiation (Rs) is [0.25 + 0.5 (n/N)] Ra, Ra is extraterrestrial radiation (mm day-1), n is actual hours of bright sunshine (hrs.), N is maximum possible hours of sunshine (hrs.) After PET is calculated on a daily basis for 50 years, it is converted into monthly data and then we calculate SPEI.
Standardized Precipitation Evapotranspiration Index (SPEI) takes both precipitation and temperature into account, it combines the response of drought to evapotranspiration. The SPEI is calculated based on the probability that the differences between precipitation and potential evapotranspiration are not exceeded, and adjusted using a three-parameter logarithmic log.it is calculated at different time scales. The SPEI at different time scales represents different climatic water balances. The difference between P and Pet for a month I is given by
Di = Pi-PETi
The empirical methods for estimating PET are compared to the standard Penman-Monteith method. The SPEI values, calculated over a 12-month timescale, facilitate drought classification into categories such as no drought (SPEI > -1), moderate drought, and severe drought. The comparison reveals that the Turc, Ritchie, and Makkink methods provide results closely aligned with the Penman-Monteith method, offering alternative approaches for drought estimation in data-scarce regions.
This study underscores the importance of selecting an appropriate PET estimation method for accurate SPEI calculation, essential for drought monitoring and water resource management. The findings highlight the versatility of SPEI in capturing drought severity over different timescales, making it a valuable tool for climate resilience strategies.
Evaluating Drought with SPEI and PET Models. (2024, Feb 21). Retrieved from https://studymoose.com/document/evaluating-drought-with-spei-and-pet-models
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