Aug 6, 2014 - India and over the entire territory of China, respectively. For other studies from the Indian subcontinent, Jhajharia et al. (2012a). © 2014 Royal ...
METEOROLOGICAL APPLICATIONS Meteorol. Appl. 22: 425–435 (2015) Published online 6 August 2014 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/met.1471
Reference evapotranspiration under changing climate over the Thar Desert in India a D. Jhajharia, * R. Kumar,b P. P. Dabral,a V. P. Singh,c,d R. R. Choudharye and Y. Dinpashohf b
a Department of Agricultural Engineering, North Eastern Regional Institute of Science and Technology, Itanagar, India Division of Agricultural Engineering, Sher-e-Kashmir University of Agricultural Sciences and Technology, Srinagar, India c Department of Biological and Agricultural Engineering, Texas A&M University, College Station, TX, USA d Department of Civil and Environmental Engineering, Texas A&M University, College Station, TX, USA e Department of Electronic Instrumentation and Control Engineering, Engineering College Bikaner, India f Department of Water Engineering, Faculty of Agriculture, University of Tabriz, Iran
ABSTRACT: Reference evapotranspiration (RET) plays a critical role in irrigation planning and is needed for the determination of water demands of crops. Thus, in the present study, trends in RET were identified over Bikaner located in the Thar Desert (Rajasthan) in India using the non-parametric Mann–Kendall (MK) test. First, RET values were estimated through the Penman–Monteith method for different time scales using meteorological data for 39 years from 1967 to 2005. Second, the effect of significant lag-1 serial correlation was removed from the time series of RET by pre-whitening. Third, trends were investigated using the MK test. RET was found to decrease significantly at Bikaner during annual, pre-monsoon, monsoon and post-monsoon time scales. On probing the causal meteorological parameters responsible for the observed RET trends in the Thar Desert, it was witnessed that wind speed dynamically influenced the observed RET changes at the annual time scale and all the four seasons over the Thar Desert. The maximum temperature, followed by relative humidity, influenced the RET trends at annual and seasonal (winter, pre-monsoon and monsoon) time scales. The calm atmosphere witnessed over this arid site substantiates RET decreases over this Thar Desert site. The results of this study support that the evapotranspiration decreases over Bikaner are controlled mainly by trends in the aerodynamic component, i.e. by the effects of significant wind speed decreases on RET, than the changes in the radiative component over the arid site located in the Thar Desert. KEY WORDS
evapotranspiration; trend; causal meteorological parameters; Thar Desert
Received 23 February 2014; Revised 17 June 2014; Accepted 18 June 2014
1.
Introduction
Water resource has become a prime concern for development and planning, including food production and flood control. The impact of climate change may be relatively severe with the reduction in the water availability over different parts of the globe. Kothawale and Rupa Kumar (2002) have reported that the all India mean annual temperature has risen at a rate of 0.05 ∘ C decade−1 during the last century, which is mostly due to the rise in maximum temperature (0.07 ∘ C decade−1 ) rather than because of the rise in minimum temperature (0.02 ∘ C decade−1 ). It is generally believed that a rise in temperature may frequently lead to a rise in evapotranspiration. Recognizing that evapotranspiration is one of the basic components of the hydrologic cycle, the Food and Agriculture Organization (FAO) adopted the concept of reference evapotranspiration (RET) in the FAO guidelines for crop water requirements based on studies by Doorenbos and Pruitt (1975, 1977), which are widely accepted for calculating evapotranspiration. RET is the phase change of water requiring a large amount of energy. Information on the amount of water required by different crops under a given set of region-specific
* Correspondence: D. Jhajharia, Department of Agricultural Engineering, North Eastern Regional Institute of Science and Technology, Nirjuli, Itanagar 791109, Arunachal Pradesh, India. E-mail: jhajharia75@ rediffmail.com © 2014 Royal Meteorological Society
climatic conditions will help greatly in irrigation planning and scheduling. RET is based primarily on measurements of radiation, vapour pressure, air temperature, relative humidity, and wind velocity over the evaporating surface. A number of researchers (Dinpashoh et al., 2011; Jhajharia et al., 2012a; McVicar et al., 2012) have analysed trends in evapotranspiration under warmer climates around the globe using data of different durations at different locations under different types of climate. Peterson et al. (1995), Chattopadhyay and Hulme (1997), Lawrimore and Peterson (2000), Golubev et al. (2001) and Roderick and Farquhar (2004) reported significant decreases in either (both) pan evaporation (Epan ) or (and) potential evapotranspiration (PET) over various parts of Russia and the United States, over India, over various parts of the United States, over a few sites in the United States and the former Soviet Union and over Australia, respectively. Similarly, several researchers from China reported decreasing trends in evapotranspiration (Chen et al., 2006; Gao et al., 2006, 2007; Xu et al., 2006; Wang et al., 2007; Zhang et al., 2007, 2009; Liu et al., 2010). Recently, Zhang et al. (2011b) reported the occurrences of decreasing annual and seasonal evapotranspiration in east, south and northwest China. Similarly, Tebakari et al. (2005), Jhajharia et al. (2009b) and Zhang et al. (2014) reported decreasing trends in Epan in the Chao Phraya River basin (Thailand), in northeast India and over the entire territory of China, respectively. For other studies from the Indian subcontinent, Jhajharia et al. (2012a)
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2. 2.1.
Materials and methods Study area and meteorological data
Bikaner (71 ∘ 54 ′ –74 ∘ 12 ′ E to 27 ∘ 11 ′ –29 ∘ 3 ′ N with altitude 237.0 m above mean sea level), is one of the important sites of the Thar Desert, located in the northwestern part of Rajasthan. The Bikaner district of Rajasthan state (India) shares its boundary on the western side with Pakistan. The Thar region is characterized by hot summers, with maximum temperature around 48.3 ∘ C, and cold winter nights with minimum temperature around 1.3 ∘ C. The average rainfall in the area is about 250 mm, and the groundwater table is also very deep (Kumar et al., 2009a). The climate of the arid regions in and around the Thar Desert has shown well-documented large-scale fluctuations and the recent regional modifications in climate are caused by the changing land-use pattern, with a large area progressively brought under irrigation. The Thar Desert consists of arid regions with not much forest cover and experiences a very high diurnal temperature range. The Thar Desert region is well-known for the high variability of annual rainfall and the highest extreme temperature range in India (Pant and Hingane, 1988). The location map of the study area is shown in Figure 1. The different meteorological data required for the computation of RET are maximum temperature (∘ C), minimum temperature (∘ C), relative humidity (%), wind speed (km h−1 ) and duration of actual bright sunshine (h). The monthly data of these climatic parameters were obtained from the India Meteorological Department (Pune), Maharashtra, for the station of Bikaner from 1951 to 2008 (Choudhary et al., 2009). The present study uses the definitions of the four seasons for Bikaner in the Thar Desert © 2014 Royal Meteorological Society
72° 00' E
74° 00' E
76° 00' E
INDIA
78° 00' E
28° 00' N
N
30° 00' N
70° 00' E
Bikaner
26° 00' N
Jaipur
24° 00' N
reported significant decreasing trends in evapotranspiration over different parts of northeast India. The RET trends may have a direct influence on the production of crops, and thus identifying trends in RET under climate change is important for understanding the effect of changing RET on agriculture. Bikaner, one of the well-known sites in the Thar Desert, has witnessed various changes due to urbanization and the extensive spread of irrigation canal networks of the Indira Gandhi Canal, carrying waters of the Satluj and Vayas Rivers through the states of Himachal Pradesh, Haryana and Punjab into the vast dry lands of the Thar Desert in the western part of Rajasthan, during the last few decades. Bikaner has witnessed significant increasing trends in minimum, maximum and mean temperatures at the annual time scale in the range of 0.1–0.4 ∘ C decade−1 during the last 58 years from 1951 to 2008 (Choudhary et al., 2009). The reported increases in temperature in the Bikaner area during this period provided the encouragement to determine if there would be any evapotranspiration increases under global warming scenarios in the arid climatic conditions of Bikaner in the Thar Desert. As no information is available about trends in RET over Bikaner, the present study was carried out with the following objectives: (1) to estimate RET using the Penman–Monteith (PM) method at annual and seasonal time scales over Bikaner under arid climatic conditions; (2) to investigate trends in RET using the Mann–Kendall (MK) non-parametric test; (3) to obtain the magnitudes of trends through the Theil–Sen test; (4) to perform trend analysis for the contributing meteorological parameters, i.e. wind speed, sunshine duration, relative humidity and temperature over the arid site located in the Thar Desert, and, (5) to identify the most dominating meteorological variables affecting RET using stepwise linear regression analysis.
0 40 80
160
240
320 Kilometers
Figure 1. Location map of Bikaner situated in Rajasthan in northwest India.
as given by Jhajharia et al. (2013) to compute the seasonal and annual time series from the monthly datasets. The four seasons were defined as: winter (January–February), pre-monsoon (March–June), monsoon (July–September) and post-monsoon (October–December). 2.2. Penman–Monteith FAO-56 method Many investigators have developed equations to estimate evapotranspiration. The most commonly used RET method, i.e. the Penman–Monteith FAO-56, was selected for the present study, because it is physically based and explicitly incorporates both physiological and aerodynamic parameters (Xu et al., 2006), and is the most reliable and universally accepted method to estimate evapotranspiration under various types of climate. The PM FAO-56 model for computing the RET is given as (Allen et al., 1998): ( ) ) ( 0.408Δ Rn − G + 𝛾 900 u2 es − ea T+273 (1) RET = ( ) Δ + 𝛾 1 + 0.34u2 where RET is the reference evapotranspiration (mm day−1 ); Rn is the net radiation at the crop surface (MJ m−2 day−1 ); G is the soil heat flux density (MJ m−2 day−1 ); T is the mean daily air temperature (∘ C); u2 is the wind speed at a 2 m height above the ground (m s−1 ); es is the saturation vapour pressure (kPa); ea is the actual vapour pressure (kPa); es − ea is the saturation vapour pressure deficit (kPa); Δ is the slope of vapour pressure versus temperature curve at temperature T (kPa ∘ C−1 ); and 𝛾 is the psychrometric constant (kPa ∘ C−1 ). Allen et al. (1998) proposed a complete set of equations according to the available weather data and time step computation, which is known as the PM method. Additional details of the PM method are available in the works of Jhajharia et al. (2004a, 2004b) and Nandagiri and Kovoor (2005). 2.3. Mann–Kendall methods for trend analysis In the present study, the non-parametric MK method was used for identifying trends in evapotranspiration and other climatic parameters, because this method is more suitable for non-normally distributed and censored data, and is less Meteorol. Appl. 22: 425–435 (2015)
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influenced by the presence of outliers in the data (Mann, 1945; Kendall, 1975). Most recently, Partal and Kahya (2006), Singh et al. (2008), Kumar and Jain (2010), Chattopadhyay et al. (2011a, 2011b), Jhajharia and Singh (2011), Dinpashoh et al. (2011, 2014), Jhajharia et al. (2012a, 2012b, 2013, 2014), Vousoughi et al. (2013) and Zhang et al. (2006, 2012) used this test for detecting trends in various climatic parameters under different types of environment. The MK test is carried out by computing an S statistic as follows: S=
n−1 n ∑ ∑
( ) sgn xj − xi
(2)
i=1 j=i+1
where n is the number of observations and xj is the jth observation and sgn(. ) is the sign function which can be defined as follows: ⎧1 if ⎪ sgn (𝜃) = ⎨0 if ⎪−1 if ⎩
𝜃>0 𝜃=0 𝜃0 S=0 S
