4.1 Annual trends in all India Evaporation:
Figure 2a shows the climatological annual cycle of the potential evaporation (Ep), actual evaporation (Ea), and its components including transpiration (Et), bare soil evaporation (Eb), interception loss (Ei), and open water evaporation (Eo) over India (Area Average) during the period 1980–2018. The annual cycle shows that there is a huge difference between potential and actual evaporation during the pre-monsoon season compared to the other seasons. The annual cycle also indicates that transpiration is the major component of actual evaporation for most of the period in a year whereas the remaining evaporation components contribute considerably during the summer monsoon season (Fig. 2a). The annual cycle (climatology) of rainfall, soil moisture, and vegetation (MODIS NDVI: 2000–2016) presented along with the actual evaporation to understand the role of climatic factors on an annual cycle of actual evaporation during the study period (Fig. 2b, 2c, and 2d). The time series plot depicts that the maximum amount of actual evaporation was observed on the last week of September (~ 2.5mm/day) whereas the maximum amount of rainfall (9-10mm/day) was observed during the second fortnight of July and the first week of August in an annual scale. The annual cycle of MODIS NDVI (vegetation) shows the maximum (0.595) amount of vegetation observed in September over India. The annual cycle of Ea clearly shows that the terrestrial evaporation is mainly driven by the vegetation and soil moisture dynamics compared to the rainfall over India.
Statistically, all India climatological annual actual evaporation is 573 mm/year with the standard deviation (σ) of 28.8 mm/year from 1980 to 2018. This shows that 51.3% of the total precipitation (1116.5 mm/year) is evaporating back to the atmosphere by different land evaporation ways including transpiration (455.9mm/year), bare soil evaporation (55.9mm/year), interception loss (33.5mm/year), and open water evaporation (27.2mm/year) (Table 1). However, the annual potential evaporation is 966.6mm/year, only, 59.3% of the actual evaporation is taking place due to the lack of surface wetness over many sub-divisions in India.
Table 1
All India mean, standard deviation (σ), and slope of potential and actual evaporation along with its components during pre-monsoon, monsoon, and post-monsoon seasons during the period 1980–2018.
| | Potential Evaporation(mm) | Actual Evaporation(mm) | Transpiration(mm) | Bare soil Evaporation (mm) | Interception loss (mm) | Open water Evaporation (mm) |
Annual | Mean(µ) | 966.63 | 573.14 | 455.90 | 55.91 | 33.54 | 27.28 |
σ | 11.86 | 28.81 | 30.43 | 8.62 | 3.44 | 0.38 |
slope | 0.16 | 1.33 | 1.91 | -0.53 | 0.16 | -0.02 |
Pre Monsoon (MAM) | Mean(µ) | 309.40 | 104.42 | 82.51 | 7.63 | 5.38 | 8.83 |
σ | 3.99 | 13.46 | 11.77 | 2.07 | 0.85 | 0.11 |
slope | 0.19 | 0.54 | 0.47 | 0.01 | 0.05 | -0.004 |
Monsoon (JJAS) | Mean(µ) | 359.65 | 270.27 | 199.89 | 37.66 | 22.87 | 9.52 |
σ | 11.84 | 10.62 | 12.41 | 5.62 | 2.24 | 0.32 |
slope | -0.27 | 0.52 | 0.85 | -0.37 | 0.08 | -0.01 |
Post Monsoon (OND) | Mean(µ) | 182.93 | 142.44 | 124.82 | 8.35 | 3.84 | 5.35 |
σ | 3.11 | 9.72 | 9.07 | 2.32 | 0.77 | 0.11 |
slope | 0.07 | 0.30 | 0.43 | -0.11 | 0.01 | -0.002 |
The inter-annual variability shows that there is an increasing trend (1.3mm/year) in all India actual evaporation even though there are no significant trends in annual rainfall or potential evaporation over India during the period 1980–2018. This increasing trend in actual evaporation is due to the significant increasing trend in transpiration (1.9mm/year) and interception loss (0.16mm/year) even though there is a significant decreasing trend in bare soil evaporation (-0.53mm/year), and open water evaporation (-0.016mm/year). The standardized anomalies (σa) are calculated during the study period and the anomalies are used to monitor the excess and deficit years in Ea during the study period. The analysis observed seven deficit (σs < 1.0) years (1980, 1985, 1989, 1991, 1992, 2002, and 2003) and eight excess (σs > 1.0) years (1998, 2006, 2007, 2008, 2010, 2011, 2013, and 2015) over India during the study period.
4.2 Spatial variability in annual Evaporation:
The spatial maps of potential & actual evaporation describe that the difference between Ep and Ea is less over highly dense-vegetated regions located in the Western Ghats and North-east India, and high over dry regions located in north-west India (Fig. 3a &3b). The sub-division-wise analysis in annual actual evaporation shows that the extremely low (< 250mm) values found over West-Rajasthan (mostly arid & semi-arid region) and Jammu & Kashmir, and low (250-500mm) amount of Ea observed over East-Rajasthan, Saurashtra &Kutch, Gujarat, Madhya Maharashtra, West Madhya Pradesh, Har. Chd. and Delhi (Haryana, Chandigarh and Delhi) sub-divisions. The high (750-1000mm) amount of Ea observed over Tamilnadu & Puducherry, Arunachal Pradesh, Assam & Meghalaya, and very high (> 1000mm) values observed over Kerala, Coastal Karnataka, and NMMT (Nagaland, Mizoram, Manipur, and Tripura) sub-divisions in India (Fig. 3b). However, moderate actual evaporation (500-750mm) values are found over the sub-divisions located in interior parts of south India, foothills of the Himalayas, and East India regions. The spatial maps of evaporation components, however, show that most of the actual evaporation is contributed by the transpiration (Fig. 3c) over most of the sub-divisions, bare soil evaporation played a significant role (~ 10–20% of Ea) in arid and semi-arid regions of India (Fig. 3d), interception loss contributed significantly (~ 10–30% of Ea) over the Western Ghats and North-East India (Fig. 3e), and open water evaporation contributed significantly (~ 10–20% of Ea) over Coastal Andhra Pradesh, Tamilnadu, and Saurashtra & Kutch regions (Fig. 3f).
4.3 Sub-division-wise trends in Annual Evaporation
Trends in annual potential evaporation, actual evaporation, and its components are presented in Fig. 4. The Ep shows a significant increasing trend in a few of the sub-divisions including west-Rajasthan, Saurashtra & Kutch, Madhya Maharashtra, Marathwada, Uttar Pradesh, and Bihar, whereas decreasing trend observed over some of the sub-divisions located in south India including Coastal Andhra Pradesh, Tamil Nadu, Kerala, and Coastal Karnataka (Fig. 4a). The actual evaporation shows a significant increasing trend over the sub-divisions located in foothills of Himalaya (Jammu &Kashmir, Himachal Pradesh, Uttar Pradesh, Bihar, Sikkim, and Gangetic West- Bengal), north-west India (Punjab, Har. Chd. And Delhi, Rajasthan, Saurashtra &Kutch, Gujarat), Western Ghats (Konkan & Goa, Coastal Karnataka, Kerala), and lower parts of South India (Tamil Nadu and South Interior Karnataka) (Fig. 4b). However, the transpiration has shown a significant increasing trend over most of the sub-divisions except Kerala, Himachal Pradesh, Uttarakhand, Chattisgarh, Odisha, Assam, and NMMT (Fig. 4c). In contrary to the transpiration, bare soil evaporation showed a significant decreasing trend over most of the sub-divisions except west-Rajasthan, Saurashtra & Kutch, Konkan & Goa, and Uttarakhand, and the increasing trend observed over western Himalaya region including Jammu & Kashmir and Himachal Pradesh (Fig. 4d). The contribution of interception loss is very minimal in most of the sub-divisions in India, whereas significant increasing trends were observed over a considerable number of sub-divisions located in the foothills of Himalaya, north-west, east, and south India regions (Fig. 4e). In the case of the open water evaporation, the sub-divisions located on the east coast of India contributed more whereas the decreasing trends were observed during the study period (Fig. 4f).
4.4 Seasonal trends in all India Evaporation
Seasonal analysis was carried out for pre-monsoon (March-May), monsoon (June-September), and post-monsoon (October-December) to understand the seasonal Ea trends over India (India as a whole). The seasonal average Ea values are 104mm (σ = 13.4mm), 270mm (σ = 10.6mm), and 142mm (σ = 9.7mm) during pre-monsoon, monsoon, and post-monsoon seasons respectively during the study period. The Mann-Kendell trend analysis shows that there is a significant increasing trend in seasonal Ea during pre-monsoon (0.53mm/year) and monsoon (0.51mm/year), whereas an insignificant (0.29mm/year) trend found during the post-monsoon season for the study period. However, the transpiration from the vegetation showed an increasing trend in all the seasons and contributed major share (pre-monsoon: 79%; monsoon: 74%; Post-Monsoon: 87.6%) during the post-monsoon season. It is also worth noting that the interception loss has shown an increasing trend during the pre-monsoon and monsoon seasons. However, the seasonal potential evaporation shows a significant increasing trend (0.18mm/year) during pre-monsoon, whereas no significant trends were observed during monsoon or post-monsoon season.
4.5 Spatial variability in Seasonal Evaporation
Figure 5 depicts the spatial variability in climatological actual evaporation, soil moisture, and rainfall over different meteorological subdivisions in India during pre-monsoon (March-May), monsoon (June-September), and post-monsoon season for the period 1980–2018. As expected, we have observed a distinct spatial and seasonal variability in actual evaporation, soil moisture, and rainfall climatology over different meteorological sub-divisions in India. However, there was a seasonal variability in evaporation, high amount of evaporation observed over dense vegetation regions (Western Ghats, North East India, and foothills of Himalayas) where the land was covered with woody Savanna, Savanna, and EverGreen Broadleaf forest; the low amount of evaporation observed over arid and semi-arid regions (northwest India) of India (Fig. 5a, 5d, 5g).
During the pre-monsoon season, the actual evaporation values are ranging between 0–1.0, 1.0-1.5, 1.5–2.5, 2.5–3.5 mm/day for 47%, 23.5%, 17.6%, and 11.7% of the sub-divisions (out of 34) respectively (Fig. 5a). The primary reason for this low evaporation rates may be less availability of soil moisture (< 0.3 m3/m3) and vegetation (NDVI < 0.38) due to the very limited (< 1mm/day) amount of rainfall in most of the sub-divisions in India (Fig. 5b, 5c). In the case of the south-west monsoon season, the actual evaporation values are ranging between 1.0-1.5, 1.5–2.5, 2.5–3.5, 3.5–4.5 mm/day for 5.9%, 70.5%, 11.7%, and 11.7% of the sub-divisions respectively (Fig. 5d). The Ea has shown tremendous improvement (> 2mm/day) during the monsoon season due to the high amount of rainfall (> 5mm/day) and soil moisture (> 0.3 m3/m3) over the Western Ghats, central, east, and northeast India (Fig. 5e, 5f). The actual evaporation is ranging between 1.5 to 2.5 mm/day for 56% of the sub-divisions during the post-monsoon season in India (Fig. 5g). It is also observed that the Ea has shown a high amount over the West and East coast of India during the post-monsoon season due to the winter monsoon rainfall (Fig. 5h, 5i).
4.6 Sub-division wise trends in Seasonal Evaporation
Figure 6 depicts the seasonal trends in Ep, Ea and its components during the pre-monsoon, monsoon, and post-monsoon season over different meteorological sub-divisions in India for the period 1980–2018 (Fig. 6). Our analysis observed an increasing trend in potential evaporation over Indo-Gangetic plains, North-West India, and West-Central India regions during the pre-monsoon season (Fig. 6a). In the case of monsoon season, a few numbers (6) of sub-divisions only showed a decreasing trend (Fig. 6b) whereas some of the sub-divisions located in west-central India showed an increasing trend during the post-monsoon season (Fig. 6c). In general, the Ep shows a decreasing trend in South India and an increasing trend in west-central India during pre and post-monsoon season.
In the case of actual evaporation, the MK test reveals that the more number of sub-divisions show an increasing trend during the pre-monsoon season compared to the monsoon and post-monsoon season (Fig. 6d, 6e,6f). The analysis also found that the significant increasing trend (p < 0.05) in pre-monsoon Ea observed over North (Jammu &Kashmir, Himachal, Haryana, and west Uttar Pradesh), North-West (Rajasthan, Gujarat, and west Madhya Pradesh), North-East (Bihar, Sikkim, and Arunachal Pradesh), South India (Coastal Karnataka, Konkan & Goa, South Interior Karnataka, Tamil Nadu, Rayalaseema, Coastal Andhra Pradesh, and Kerala) regions (Fig. 6d). The sub-division wise Sen’s slope values were > 1 mm/season for four sub-divisions in South India [Coastal Karnataka (2.35mm/season); Kerala (1.9 mm/season); South Interior Karnataka (1.4 mm/season)] and two sub-divisions in foot-hills of Himalayas (Har. Chd. & Delhi and SHWB & Sikkim) during the pre-monsoon season.
In the case of monsoon season, an increasing trend in Ea was observed over semi-arid regions (West & east Rajasthan, Rayalaseema, Tamil Nadu, and SI Karnataka) and north & east India (Uttar Pradesh, Bihar, Jharkhand, and Gangetic West Bengal) (Fig. 6e). The Sen’s slope also reveals that the sub-divisions located in arid and semi-arid regions reported the increasing trend of > 1mm/season during the study period. The analysis observed an increasing trend in Ea over foot-hills of the Himalaya and Saurashtra & Kutch regions during the post-monsoon season (Fig. 6f). The study has not observed any decreasing trends in actual evaporation during the study period.
The present study observed a significant difference between potential and actual evaporation trends over India during pre-monsoon, monsoon, and post-monsoon seasons. To understand the reason for the increasing trend in Ea, the seasonal trends are calculated for the major evaporation components including transpiration, bare soil evaporation, interception loss, and open water evaporation during the study period. The transpiration and interception loss from the vegetation have shown a tremendous increasing trend in the sub-divisions where the Ea has shown an increasing trend during the pre-monsoon season (Fig. 6g, 6m). During the monsoon season, even though there is an increasing trend in transpiration (Fig. 6h) and interception loss (Fig. 6n) over most of the sub-divisions in India (where the major crop activities are taking place in plain areas), the Ea has shown an increasing trend (Fig. 6e) in a very few sub-divisions in South India, Northwest India, and East India due to the significant decreasing trend in bare soil evaporation (most of the crop growing areas; Fig. 6k)) and open water evaporation (Coastal Andhra, Telangana, NI Karnataka, Madhya Maharashtra, Marathawada, Gujarat, Saurashtra & Kutch, East Rajasthan, and Uttarakhand; Fig. 6q)). During the post-monsoon season, a very few sub-division which are located in foot-hills of Himalaya shown an increasing trend in Ea (Fig. 6f) due to the increasing trend in transpiration (Uttar Pradesh, Bihar, Sikkim; Fig. 6i)) and open water evaporation (Punjab and Jammu & Kashmir; Fig. 6r)).
4.7 Seasonal Trends in Climatic parameters
Seasonal Trends in climatic parameters comprising of soil moisture (SM), rainfall (RF), surface net solar radiation (SSR), surface temperature (T), wind speed (WS), and relative humidity (RH) were also computed for the study period (1980–2018) to understand the role of climate on actual evaporation anomalies at sub-division scale in India (Fig. 7). Similar to the actual evaporation, significant trends in soil moisture were observed over North-West and south India regions during the pre-monsoon season (Fig. 7a). It was also observed that the pre-monsoon rainfall has shown a significant increasing trend over Western-Ghats (Coastal Karnataka, Konkan & Goa), Kerala, SI Karnataka, and Rayalaseema sub-divisions in India during the period 1980–2018 (Fig. 7d). The decreasing trends in pre-monsoon rainfall were observed over hill regions including Jammu& Kashmir, Himachal Pradesh, and Arunachal Pradesh (Fig. 7d). The temperature (Fig. 7g) has shown a significant increasing trend over northwest and north-east India whereas surface solar radiation (Fig. 7m) shows a decreasing trend in south peninsular India during the pre-monsoon season. The pre-monsoon relative humidity (Fig. 7j) has shown an increasing trend in some of the sub-divisions in south India (Kerala, Coastal Karnataka, South Interior Karnataka, Telangana) and north India (Uttar Pradesh, Bihar, and Sikkim). The analysis does not observe significant trends in pre-monsoon wind speed while decreasing trends observed over Kerala, Jharkhand, and NMMT sub-divisions (Fig. 7p).
In the case of monsoon season, similar (increasing) trends like Ea has observed in soil moisture (Fig. 7b) & RH (Rajasthan; Fig. 7k)), and temperature (Tamil Nadu, Kerala, Coastal Karnataka, SI Karnataka, east UP, Bihar, Jharkhand, and Gangetic West Bengal; Fig. 7h) during the study period. However, the net surface solar radiation has shown significant decreasing trends over northwest India and southeast India (Fig. 7n). The rainfall has shown decreasing trend in east Uttar Pradesh, Bihar, and Assam sub-divisions from 1980 to 2018 (Fig. 7e).
The trends in climatic parameters during the post-monsoon season shows that the RH has similar (increasing) trends similar to Ea over the sub-divisions located in foothills of the Himalayas (Uttar Pradesh, Bihar, and Sikkim) (Fig. 7l), whereas the rainfall shows inverse trends over Jammu &Kashmir, Uttar Pradesh, and Arunachal Pradesh (Fig. 7f). The trend analysis also observed decreasing trends in net surface radiation over the southwest coast, Tamilnadu, and east India (Fig. 7o). The surface temperature has shown an increasing trend over most of sub-divisions India except East India, north interior Karnataka, Vidarbha, and Jammu& Kashmir (Fig. 7i).
4.8 Relationship between Ea and Climatic parameters
Correlation analysis was carried out between the actual evaporation and the climatic parameters for pre-monsoon, monsoon, and post-monsoon seasons during the study period (Fig. 8). The correlation coefficients show that the actual evaporation has a strong (99% significant level) positive correlation with rainfall (Fig. 8d), soil moisture (Fig. 8a), and relative humidity (Fig. 8j), opposite correlation with SSR (Fig. 8p) over most of the sub-divisions in India (except northeast India, Jammu & Kashmir, and Himachal Pradesh) during pre-monsoon season. However, temperature (Fig. 8g) showed negative correlations over most of the sub-divisions and positive correlations over Jammu& Kashmir and Arunachal Pradesh during the pre-monsoon season.
The analysis observed a complex relationship with climatic parameters during the monsoon season. In general, rainfall (Fig. 8e), soil moisture (Fig. 8b), RH (Fig. 8k) have a positive correlation over the sub-divisions located in water-limited areas (i.e. South and north-west India) and a negative correlation found over energy-limited areas located in east India. In contrast to the rainfall, net surface radiation shows a significant positive correlation over energy-limited areas and a negative correlation over water-limited areas in India (Fig. 8q). Wind speed has shown a strong negative correlation over the arid and semi-arid regions of India (Fig. 8n).
In the case of the post-monsoon season also soil moisture, rainfall, and humidity showed a significant positive correlation with Ea over most of the sub-divisions in India except foothills of Himalaya regions (Fig. 8c, 8f, 8l). Net surface radiation shows a strong negative correlation over most of the subdivisions in India whereas a positive correlation is observed over western Himalaya (Jammu &Kashmir and Himachal Pradesh) and Arunachal Pradesh (Fig. 8r).