3.1. Changes in Growing Season Length and Shifts in the timings of Crop Growth Phenological Phases
Our results show that the simulated timing and length of phenological phases shifted and decreased considerably during both cropping season with prominent shift for wheat in the rabi season as shown in case of Punjab Pakistan (Fig. 2).
Fig. 2 Shifts in the timing of the crop phenological phases (vegetative (Veg), reproductive (rep) and ripening (Rip)) during the growing season length estimated for three different years for wheat (a) and rice (b) as shown by solid, dashed and point-dashed curves. The related horizontal and vertical lines show variations in the range of the three phenological development phases. Growing season length (GSL) trends for wheat (c) and rice (d) over six study sites (i.e. Punjab Pakistan, Punjab India, Haryana, Uttar Pradesh, Terai-NP and DRBR-BD) during periods 1981-2100
The different crop growth phases (i.e., vegetative, reproductive, and ripening) occurred earlier in the growing season. Moreover, the length of each crop growth phase also decreased noticeably in future.
Fig. 2 c to d show the growing season length patterns of both wheat (rabi season) and rice (kharif season) over the six study sites during the whole study period (1981-2100). The decrease in growing season length is large for wheat in the rabi season (i.e., on average 15-20 days) as compared to the decrease in growing season length of rice during the kharif season (i.e., on average 8-12 days) over 120 years for all study sites. The growing season of wheat is longest in Punjab India and Haryana. The growing season of rice is longest in Terai region of Nepal and the selected districts of Bangladesh.
The shift in the timing of crop growth phenological phases and changing growing season length are directly linked with the location and season-specific temperature patterns in the region. These changes have a significant impact on crop and phase-specific irrigation water demand and supply patterns in the region as presented in the following sections.
3.2. Irrigation Water Demand by Crops during Sensitive Crop Growth Phases
The irrigation water demand of wheat and rice during vegetative and reproductive growth phases in Punjab Pakistan is given in Fig. 3.
Fig. 3 Inter-annual variations and trends of irrigation water demand (mm) of wheat (a) and rice (b) during the vegetative and reproductive crop growth phases for the period 1981-2100 for Punjab Pakistan using the ensemble mean of four GCM’s of RCP4.5-SSP1 emission scenarios
For wheat in Punjab Pakistan (see Fig. 3), the irrigation water demand during the control period (1981-2010) is on average almost equal in both crop growth phases i.e. (80 ± 4.4 mm) and (77 ±1 mm). During 2011-2100 period, there is only a slight increase projected in water demand during the vegetative phase. In contrast, for the reproductive phase there is a relatively large decrease projected in wheat water demand. These changes lead for both crop phases combined to a total decrease in future water demand for wheat. For rice the trends are in the opposite direction. A decrease of the water demand in the vegetative phase and an increase in the reproductive phase. For both wheat and rice the reproductive phase shows the strongest erratic behavior between years. The irrigation water demand gap during both crop growth phases will be widened in the future (especially towards the end of the century). These crop phase-specific changes in irrigation water demand are strongly linked with patterns of phase-specific temperature and precipitation projections (Supplementary Table 1).
Fig. 4 shows the spatio-temporal distribution of irrigation water demand statistics during sensitive crop growth phases of wheat and rice for all selected study sites. Our analysis revealed that on average ~ 60% of the total irrigation water demand i.e., 114 mm out of 164 mm for wheat and 249 mm out of 321 mm for rice is needed during the vegetative and reproductive crop growth phases in almost all study sites (Supplementary Table 2a). Supplementary Table 2b revealed similarities among most study sites in particular the direction of changes in a season, but with a variable degree of magnitude between locations. For example, consistent with our results in Punjab Pakistan (Fig. 3), the irrigation water demand patterns in other study sites will also increase during the reproductive phase of rice (26 mm ± 19 mm, ranging from 0 to 45 mm), and for the vegetative phase of wheat (6 mm ±1.04 mm, ranging from -1.67 to 15 mm except for selected districts of Bangladesh where a slight decrease is projected). During the vegetative phase of rice, a decrease (11 mm ± 19 mm, ranging from -40 to 18 mm) is predicted, with the exception of Uttar Pradesh (Supplementary Table 2c). These statistics are estimated for the selected study sites where changes are aggregated over six study sites for period 2011-2100. In Uttar Pradesh a significant increase of 0.27 mm per year is found. During the reproductive phase of wheat, the substantive decrease (13 mm ± 9 mm on average, ranging from -13 to -1.1 mm) in irrigation water demand is quite uniform across all study sites (varying from -0.01 mm to -0.28 mm per year). This decrease could be linked with more water becoming available from rain and snow and glacier melt (See negative correlations in Supplementary Table 3).
For rice during the kharif season, the irrigation water demand varies largely between study sites which are directly linked with monsoon patterns (quantity and timing) in the region. For example, the irrigation water demands are large for the western study sites of the IGB river basins i.e., Punjab Pakistan, Punjab India and Haryana. Whereas, relatively low irrigation water demands are estimated for the eastern study sites i.e., Utter Pradesh, Terai-NP and DRBR-BN where ample amount of water is available from rain (See mean precipitation for rice in Supplementary Table 1b).
Consistent with the long-term trends, the analysis of 30-year means revealed a gradual progression of trends over the four periods (1981-2010, 2011-2040, 2041-2070 and 2071-2100) (Fig. 4).
Fig. 4 Projected 30 years mean irrigation water demand (mm) for wheat (a-b) and rice (c-d) during the vegetative (a, c) and reproductive (b, d) crop growth phases for six study sites during the period 1981-2010, 2011-2040, 2041-2070 and 2071-2100 using the ensemble mean of four GCM’s of RCP4.5-SSP1 emission scenarios
The projected change in irrigation water demand is consistent among the four periods during both crop phases of wheat and the reproductive phase of rice in all study sites (Fig. 4 a-b). Whereas, these 30-year mean values show less consistency during the vegetative phases of rice during the kharif season (Fig. 4c). These latter changes are more pronounced especially during the second half of the century (Fig. 4). Our results show that the shifts in phase-specific irrigation water demands are more prominent in case of RCP8.5-SSP3 as compared to the RCP4.5-SSP1 scenario (Supplementary Table 4).
3.3. Contribution to Irrigation Water Supply by Surface Water and Groundwater during Sensitive Crop Growth Phases
Crop phase-specific irrigation supply from surface water and groundwater show different contributions and varying patterns in Punjab Pakistan (Fig. 5).
Fig. 5 Inter-annual variations and trends of irrigation supply (mm) by sources (surface water and groundwater) during the vegetative (a, c) and reproductive (b, d) phases of wheat (a-b) and rice (c-d) for the period 1981-2100 for Punjab Pakistan simulated by LPJmL using the ensemble mean of four GCM’s of RCP4.5-SSP1 emission scenario
Irrigation supply from surface water is two to four times higher than from groundwater during both crop phases of wheat and rice for the control period (1981-2010). In future, the contribution of surface water supply increases most prominently during the vegetative phase of wheat and the reproductive phase of rice (Fig. 5 a, d). Only for the reproductive phase of rice there is a prominent increase in groundwater use (Supplementary Table 5-I). These changes are mainly associated with the changing precipitation patterns in certain crop phases (See precipitation trends in Supplementary Table 1b).
The rate of increase in surface water during the reproductive phase of rice is less than the rate of decrease during the vegetative phase (Supplementary Table 5), indicating an increasing pressure on groundwater during the latter part of the kharif season in all selected study sites of IGB river basins. The largest change i.e. 0.12 and -0.27 mm phase-1 year-1 in surface water is observed in Uttar Pradesh and Punjab Pakistan during vegetative and reproductive phases of wheat respectively. Whereas, for rice, the largest change i.e. -0.32 and 0.31 mm phase-1 year-1 is observed in Punjab India and Punjab Pakistan during vegetative and reproductive phases for period 1981-2100 indicating a varying surface water supply in region and seasons under a changing climate.
Similar to the irrigation water demand changes, irrigation supply by surface water and groundwater show increases during the vegetative phase of wheat (5 mm and 1 mm) and reproductive phase of rice (11 mm and 15 mm) with a substantive decrease during the reproductive phase of wheat (5 mm and 8 mm) and the vegetative phase of rice (6 mm and 5 mm) (Supplementary Table 5-II).
Fig. 6 Projected 30 years mean irrigation supply (mm) by sources (surface water and groundwater) during the vegetative (a, c) and reproductive (b, d) phases of wheat during rabi season (a, b) and of rice during kharif season (c, d) for six study sites for the periods 1981-2010, 2011-2040, 2041-2070 and 2071-2100 using the ensemble mean of four GCM’s of RCP4.5-SSP1 emission scenarios
Fig. 6 (a-d) shows the spatial distribution and quantitative contribution of irrigation supply from surface water and groundwater during the vegetative and reproductive growth phases of wheat and rice for the selected study sites in IGB river basins. Surface water irrigation is largely used in Punjab Pakistan and Uttar Pradesh. While groundwater is also a very important source of irrigation and plays a significant role in Punjab India, Haryana and Uttar Pradesh providing about 56%, 77% and 30% of crop water demand during the vegetative phase and 61%, 87% and 44% during reproductive phase respectively. Phase-specific irrigation supply by sources is negligible in Terai-NP and DRBR-BN which is associated with the fact that in the eastern part of the IGB river basins monsoon rains are sufficient to fulfil the rice water demand and no irrigation is needed. However, during the rabi season, additional water is required from groundwater to fulfil the irrigated wheat water demand. The phase-specific estimates of irrigation supply by surface water and groundwater are relatively large under the RCP8.5-SSP3 scenario (Supplementary Table 6).
3.4. Future changes in Spatial Distribution of Irrigation Supply from Groundwater during Sensitive Crop Growth Phases
For wheat cultivated areas in the IGB river basins, Fig. 7 shows future changes in irrigation supply from groundwater relative to the control period (1981-2010, see Fig. S1(a-b)).
Fig. 7 Projected changes in irrigation from groundwater (mm) during the vegetative (a, c, e) and reproductive (b, d, f) phases of wheat over the whole IGB river basins for three reference periods i.e., 2011-2040 (a-b), 2041-2070 (c-d), and 2071-2100 (e-f). Changes are relative to the 30 years mean groundwater supply for the control period 1981-2010 (Fig. S1a). Positive values indicate the groundwater supply is increasing relative to the control period
During the vegetative phase, a positive change, indicating an increase in groundwater required for irrigation, is concentrated in the Southwestern part of the Ganges basin and in the Central Indus. The biggest change in groundwater demand in the vegetative phase is expected to take place in the first period 2011 – 2040 (Fig. 7a). In the two later periods the changes are not so prominent (Fig. 7b and 7c). During the reproductive phase of wheat, a negative change in irrigation groundwater demand is large in extent and takes place in all three periods, as shown by the blue color in Fig. 7b, 7d and 7f. These decreases in future groundwater demand are mainly attributed to the increased winter precipitation in the Eastern parts of the IGB river basin. This decrease in future irrigation from groundwater is projected for most parts of the IGB basin, except for the areas of the Central Indus and South Ganges. For these two areas, the change in groundwater supply relative to 1980-2010, is about 40 to 60 mm in 2011-2040 and reduces to 10 to 40 mm in 2071-2100. Comparing the vegetative phase with the reproductive phase shows a possible balance in the need of groundwater between the two phases, but also implies an earlier need of groundwater in the vegetative phase.
Fig. 8 Projected changes in irrigation from groundwater (mm) during the vegetative (a, c, e) and reproductive (b, d, f) phases of rice over the whole IGB river basins for three reference periods i.e., 2011-2040 (a-b), 2041-2070 (c-d), and 2071-2100 (e-f). Changes are relative to the 30 years’ mean groundwater supply for the control period 1981-2010 (Fig. S1b). Positive values indicate the groundwater supply is increasing relative to the control period
For rice, the irrigation need from groundwater is expected to increase (> 60 mm) in future (See yellow and red color in Fig. 8a and 8b). Especially in the reproductive phase a continuous increase over the three periods is observed.
During the vegetative phase a large increase (> 60 mm) is projected in the first period 2011-2040 (See the Ganges basin in Fig. 8a), but this increase is becoming weaker, and in some areas even changes towards a relative decrease in the periods 2041–2070 and 2071-2100 (See Fig. 8c and 8e). In the Eastern Indus area (e.g. Punjab India) a decreasing trend in groundwater demand during the vegetative phase is followed by an increasing demand during the reproductive phase. These trends are projected to increase in quantity and extent to Punjab Pakistan and Haryana by the end of the century. This change in groundwater demand may imply the need to store water in the vegetative phase, to be used in the reproductive phase.
During the reproductive phase of rice, the largest positive increase in groundwater demand, i.e. > 60 mm, is projected for the Indus basin. Also, the Central-Eastern part of the Indus and the central part of the Ganges basin with exceptions in some Southern parts of the Ganges basin show a continuous positive increase (See Fig. 8b, 8d and 8f). The continuous increase in future groundwater demand for irrigation during the reproductive phase of rice in most areas of the IGB river basins is attributed to increasing weakening of the monsoon rains during this period.
In the central and Southern parts of the Indus, the groundwater demand is projected to shift from the vegetative phase towards the reproductive phase. This shift will intensify at the end of the century. In the Ganges basin, the change in groundwater demand will take place in the period 2011-2040 in the vegetative phase. This change will cause a shift towards an increased demand in the reproductive phase in the period 2071-2100 for the Ganges basin.