1. Introduction
South Asia, including India, Pakistan, Nepal, and Bangladesh, is one of the most densely populated agrarian regions in the world. Agriculture provides 70% of the livelihood in India [
1] and 66% in Pakistan [
2] with more than 45% of the land is already in use as cropland [
3]. Multiple and double cropping systems are often used in the region to produce the desired food demand during the two main cropping seasons i.e., kharif (June–September) and rabi (November–March) with wheat (
Triticum aestivum) as a major crop sown in rabi and rice (
Oryza sativa) in kharif [
4,
5,
6]. Agriculture contributes 21% of the gross domestic product (GDP) in Pakistan [
7], 14% in India [
8], 29% in Nepal and 16% in Bangladesh. The agriculture sector consumes up to 91% of the total water use in South Asia [
9]. Water availability and demand in the region are highly variable within and between the years [
10,
11]. Water availability in the region is dominated by monsoon rainfall, which originates from the Bay of Bengal and ends in the western part of South Asia, and accounts for 60–90% of the total annual precipitation [
12]. India receives approximately 87% of its annual total precipitation during the monsoon season [
13], Pakistan receives 55–60% [
14], Bangladesh 72% [
15] and Nepal 80% [
16]. This spatiotemporal gradient of precipitation determines the seasonal distribution of irrigation water availability and demand in the region [
4].
The irrigation system in the Indo-Gangetic Plain is the World’s largest water system on Earth [
17], encompassing more than 40% of the total cropped area in the region [
18]. Up to 70% of the total agricultural production in India is from irrigated land [
19]. Agricultural production in Pakistan is largely dependent on irrigation water [
20] and consumes 90% of the water withdrawals for food production [
21]. The surface water and groundwater resources are commonly used for irrigation purposes in the region [
22]. A large part of the kharif season crop water demand in Bangladesh, India and Nepal is fulfilled by monsoon rainfall. Whereas, agricultural production in the rabi season is mostly supported by groundwater extractions [
23]. Groundwater extraction is largest in India exceeding 230 km
3 annually and about 85% of these withdrawals are used for agricultural production [
9,
22,
24]. However, in Pakistan, most of the agriculture production during the kharif season depends on water either from snow and glacier melt or from groundwater resources, whereas, during the rabi season, it mostly depends on groundwater extraction [
4,
25,
26,
27]. Hence, any changes in the monsoon onset or the meltwater cycle can cause serious problems to food production in the region [
28]. Human interventions (irrigation water withdrawals) have already caused discernible impacts on the hydrological cycle [
29]. On top of that, land use change and climate change have revealed additional negative impacts on the agricultural production in most of the arid and semi-arid regions of South Asia [
30,
31,
32,
33]. Moreover, the region is already water-stressed and declared as a climate change hotspot [
34].
An increase in climate variability (natural variations (changes) in the climate variables) within and between years is inevitable, with substantial shifts in monsoon onset, precipitation frequency, intensity and, hence, change in the water cycle in South Asia [
35]. Increased climate variability not only causes floods and droughts, and changes water availability patterns, but also affects net crop production by influencing biochemical and metabolic processes [
36,
37]. In some regions, extreme high temperatures have a strong negative impact on crop growth and development [
38,
39,
40,
41] thus reducing yields significantly by shortening of the growing season length. Yield loss is even larger if extreme temperatures (both high and low) coincide with sensitive crop growth phases e.g., the reproductive phase [
42,
43,
44,
45]. Water stress (droughts or floods) is another critical climatic driver, which may cause major crop yield loss in Asia [
46,
47]. As inter-annual variations in crop yields are largely impacted by climate variability [
48,
49], detailed knowledge of crop yield sensitivity to climate variables and associated changes in crop water demand are crucial to assure a sustainable and increased future food supply in the region [
50,
51].
Multiple-cropping patterns and rapidly growing technological advancement in the field of agronomy have increased the crop production many folds in the region [
52] However, to achieve a further sustainable increase in crop yields, implementation of long-term appropriate adaptation measures are required to reduce crop yield vulnerability to changing climate [
48] Various studies have been conducted to assess the impacts of changing climate on irrigation water availability and use by crops [
53,
54]. A few studies have focused on estimating the crop specific seasonal irrigation water demand in South Asia [
4]. Moreover, studies on crop yield sensitivity to int(e)r(a)-annual variations in climate variables and associated changes in crop water demand in South Asia are still limited. Therefore, our main research question is: “To what extent is the variation in irrigated crop yields and irrigation water demand determined by int(e)r(a)-annual climate variability?”. Our hypothesis is that variability in yield and water demand strongly depends on climate variations in the most sensitive crop growth phases. To analyse this, the following sub-questions are addressed:
In this study, the relationship of irrigated crop yield and irrigation water demand sensitivity to climate variables at seasonal (sowing to harvest) and crop phase specific scale have been estimated in the Indus, Ganges and Brahmaputra (IGB) river basins. We used observed yield statistics and a crop-water model (LPJmL) for our analysis.
4. Discussion
The climatic variations play a vital role in crop development and growth but show a varying degree of relationship strength by season and location. A number of theoretical, modelling and empirical studies have already estimated the impacts of climate variability on crop yield fluctuation using different methodology and datasets at annual, seasonal and regional to national scales [
39,
98,
100]. These studies suggested a range of climate variability impacts on crop production. For example, a recent study reported that approximately 33% variations in observed global yields are caused by inter-annual climate variability. Whereas, in some agriculture intensive worldwide areas at national scale, >60% yield variability is linked with temperature and precipitation variations [
80]. Estimation of crop yield sensitivity to climate variability depends on several factors i.e., spatial and temporal scale, data length for analysis, methods used etc. It should be noted that crop yield variations can also be caused by other non-climatic parameters i.e., irrigation scheduling, land-use conditions, edaphic variables (e.g., soil characteristics), crop varieties, pests etc. [
101,
102]. The impact of these variations are not taking into account in our simulations with the LPJmL model, but are inherent part of the observed yield data and should have been consider to compensate the negative impacts of climatic conditions [
103]. Integrated crop water assessments at different spatial scales are crucial to guide farmers, researchers, stakeholders and policy maker to understand, investigate and plan sustainable strategies of crop production in climate hotspot countries [
104]. In our study, we analyzed the crop yield relationships with climate variables at low spatial and temporal resolution often used by policy makers. By using a higher resolution both in space and time we demonstrated the impact of climate variability during sensitive crop growth phases on yield. These higher resolution results may better prepare framers and water managers for increased climate variability than the coarser resolution impact assessments for policy makers. In the next sections we discuss the sensitive crop growth phases and the impact of climate variability on yield and water demand. The last section discusses the limitations of our study.
4.1. Crop Yield Sensitivity at Different Spatial Scales
Our results of reported yield’s sensitivity to climate variables revealed that temperature and precipitation are not the only drivers to cause any substantial change in crop yield production. At larger spatial scale (i.e., province and states), both observed wheat and rice reported yields show a weak relationship (up to 21%) with seasonal precipitation and even weaker links with temperature (
Table 2). At coarser spatial scale i.e., province and states, weak correlations between crop yields and climate variables may be linked with averaging out of the climatic variations. Crop yield variations showed relatively stronger relationships (up to 30%) with climate variables when investigated at a smaller spatial scale i.e., districts in Punjab province Pakistan (
Table 3). At district scale, both crops, particularly wheat, show strong association with temperature variations. The low correlations and flat slopes of the observed data at province level clearly show the influence of other non-climatic factors and the averaging out effect of variations as to be expected at this coarse resolution (see
Table 2). Although, the correlations are low, the direction of the slopes of observed data and simulated data are in the same direction at the province level.
Santiago et al. [
103] also reported crop yield fluctuations are responding stronger to temperature trends i.e., up to 12%, than precipitation trends i.e., up to 2% (in case of wheat crop yields, see
Figure 2 of [
103]. In this study, the authors used panel models to evaluate the impact of growing season precipitation (P), average temperature (T) and diurnal temperature range (DTR) on historical period yield trends in wheat, maize and soy crops from 33 counties of the Argentine Pampas region in South America. The stronger impacts of temperature variations on rice yield variation, as compared to precipitation, has also been reported in Bangladesh. The limited impact of precipitation variations are caused by the already high water availability and irrigation use [
80]. The variations in the strength of the correlations between yield and climate variables varies between regions (i.e., districts, province and states). This variation may be associated with local climate conditions (thresholds and patterns), crop variety used and size of the area under consideration for analysis. Poor correlation values −0.14 to 0.13 (
Table 2) could also be caused by averaging out location and time specific variations.
Our modelling results show that crop yield variations are strongly associated with climatic variations with a statistically strong relationship (
p < 0.001) when estimated at grid cell (aggregated over the study sites) scale (
Table 5). Strong correlations of modelled yield and climate variables at province and state level are associated with the fact that temperature and precipitation are the main drivers in our modelling setup to cause any change in crop production. In our modelling setup, the irrigation system (i.e., surface irrigation), crop sowing dates and any management and adaptation options (irrigation efficiency, crop variety, pesticides etc.) remained constant. Hence, a statistically relatively strong relationship of simulated wheat and rice yields is observed with both temperature and precipitation. Our results indicate that 27–72% variations in wheat yields and 17–55% variations in rice yields are linked with temperature variations. The correlation strength varies from one location to another which might be associated with the size of the irrigated land in the selected study sites.
Continuous higher temperatures and precipitation variability throughout the wheat and rice crop growing seasons can negatively affect crop production [
82]. Studies estimate that 3–10% (4–5 million tons) wheat yield loss in the Southern and Eastern parts of Asia are linked with 1 °C rise in temperature [
87,
103]. Similarly, increasingly higher temperatures caused a huge loss in rice crop production in the Indo Gangetic Planes and Sri Lanka [
95,
105].
The impacts of precipitation variations on crop yields are generally compensated by supplying water as irrigation [
4]. In our modelling results, up to 39% variations in wheat and up to 75% variation in rice yields in six study sites are linked with precipitation variations in the absence of additional water supply. Relatively weak correlations of wheat (PunjabP, PunjabI and Haryana in Indus) and rice (Uttar Pradesh, Nepal and Bangladesh in Ganges and Brahmaputra river basins) yields with precipitation variations results from the yield dependency on irrigation application. Wheat crop production in the IGB river basins is mainly irrigated, and therefore, shows a weak non-significant relationship with precipitation in most of our study sites. However, for rice crop, during the kharif season, an ample amount of water is available from precipitation in the IGB river basins. This seasonal precipitation pattern leads to a relatively strong and significant (
p < 0.001) relationship of rice yield with precipitation in our study sites. The strength of these correlations depends largely on the local climatic and soil conditions which varies with location from East to West and between seasons.
Our statistical analysis results revealed that the crop yield variations are associated with climate variables with much stronger correlations at higher spatial scales i.e., at grid cell level. This is depicted by the high correlation values up to 72% variations in wheat yield and up to 55% variations in rice yields influenced by temperature and up to 39% variations in wheat and up to 75% variations in rice yields by precipitation in the selected study sites. Our modelling analysis results also revealed that temperature is the stronger driver to cause changes in both wheat and rice yields variations in the IGB region.
4.2. Climate Sensitive Crop Growth Phases
Increased climate variability particularly higher temperatures during sensitive crop growth phases can affect net crop production negatively by influencing biochemical and metabolic processes [
37,
106]. Also, water shortage during the flowering phase causes severe damages to rice yield by affecting the dry matter allocation to the harvestable storage organs [
71,
107,
108]. Our literature-based analysis identifies that both wheat and rice yields are most sensitive to temperature and precipitation during the vegetative and reproductive crop growth phases (
Table 4). Wheat yields show more sensitivity to temperature variations during both phases. Whereas, rice crop yields show stronger sensitivity to water stress during reproductive crop growth phase.
Studies revealed that crop exposure to heat and or water stress during the vegetative phase can lead to reduced crop production and poor grain quality. The reproductive phase including flowering/anthesis sub-stage is generally known as the more sensitive phase to temperature stress and can lead to irreversible loss in crop production [
50,
91,
108,
109,
110,
111,
112]. Any extreme event (heat and or water stress) duirng these sesnsitive crop growth phases can have major implication on net crop production [
82,
89,
113].
Our modelling results are generally in line with the published studies given in
Table 2, where both wheat and rice crops show strong correspondence with the vegetative and reproductive phase climate variables. Further, our analysis revealed that both crops show a stronger relationship with temperature and precipitation during the reproductive phase. We also observed that other than vegetative and reproductive phases, both wheat and rice crops show significant, but fluctuating relationships (i.e., r from −0.35 to +0.53 for wheat and r from −0.53 to +0.41 for rice) with ripening phase precipitation for some states. These fluctuations could be associated with location and season specific climate and crop conditions. Similarly, Vijay et al. [
114] also found the milk stage in the ripening phase as the more sensitive crop growth phase in wheat which can lead to reduced crop production. Our modelling results of crop yield sensitivity to phase-specific climate variables (based on Pearson correlation coefficient) suggest the need for time and location specific adaptation and management to cope with the uncertain climate variations. The sensitivity of crop yields with crop phase-specific temperature and precipitation varies differently in all study sites and seasons. The ranges of these variations correspond to the region-specific temperature and precipitation conditions that vary geographically [
43].
4.3. Impacts of Climate Variables on Irrigation Water Demand during Sensitive Crop Growth Phases
Interannual climate variability is evident and its impacts on water availability in soil and plants are obvious globally with a substantial influences on irrigation water demand and supply in arid and semi-arid areas of South Asian countries [
67,
115,
116,
117]. Irrigation also plays a major role in cooling soil temperatures during crop’s exposure to high temperatures [
118]. The crop development and climate conditions determine the irrigation water demand in certain crop growth phases.
Our results of crop phase-specific (vegetative and reproductive) irrigation water demand by both wheat and rice show a strong and a varying relationship with phenological phase-wise temperature and precipitation (
Figure 4). Irrigation water demand by wheat crop show mixed (mainly positive correlations during the vegetative phase and both positive and negative correlations during the reproductive phase) relationship with temperature (
Figure 4a). Whereas, irrigation demand by rice crop mainly present a positive correlation with temperature in all study sites during both vegetative and reproductive phases (
Figure 4c). The strong positive correlations during both vegetative and reproductive phases of rice crop during kharif season could be associated with the higher seasonal temperatures (
Figure 4c). An increase in temperature causes a rise in evapotranspiration which will require higher irrigation water demand during different developmental phases. Crop phase-specific irrigation requirements depend on the length of crop growth stages which are ultimately related to temperature conditions [
38,
39]. However, irrigation water demand is generally negatively correlated with precipitation during both vegetative and reproductive phases of wheat and rice crop in all study sites (
Figure 4b,d). An increase in precipitation reduces the crop water requirement from irrigation. A substantive decrease of 17 % in global irrigation water demand is reported due to the beneficial effects of CO
2 on plants, shortening of growing season length linked with climate change (warming) and regional precipitation increases [
99]. The crop phase-specific correlations of climate variables with irrigation water demand by crops vary largely from East to West in seasons. For example, during vegetative phase of wheat crop, temperature is positively correlated with irrigation water demand in Punjab Pakistan. However, vegetative phase temperature show negative relationship with irrigation water demand by wheat crop in Bangladeshi districts. Similarly, precipitation during reproductive phase of rice crop show large range of correlation with irrigation water demand. Large range of correlation values of irrigation water demand with temperatures and precipitation are the result of the large-scale spatial distribution of monsoon precipitation patterns in the region [
119]. Temperatures show strong impacts on irrigation water demand by rice crop during the kharif season with stronger and direct relationship during the reproductive phase (
Figure 4c). Whereas, precipitation shows strong impacts on wheat crop irrigation demand (
Figure 4b). During the crop growth period, when temperatures are well above optimal temperatures, it accelerates the crop maturity process and reduces net irrigation water demand [
39].
To understand the impacts of intra-annual climate variability on crop yields, a good insight into the crop growth phase specific irrigation water demand and supply by different water sources will help in determining the potential adaptation options for sustained future food security.
4.4. Limitations of the Study
A number of uncertainties in our results (different correlations strengths of crop yields and climate variables using observed and modelled data at province level) can be associated with the model, data and methodology used. The model simulation are reasonably good on average [
4,
120], however, year- to year variability needs further improvement. Considering observed climate and yield data unavailability at higher spatial and temporal scales, we have used LPJmL model simulated data to estimate the relationships of climate variables with yield and water demand at higher spatio-temporal scale (i.e., grid-cell and crop growth phases). In the current simulations, climate variables are the main drivers to cause year to year yield variations with fixed land-use information, standard field/crop management practices, irrigation system (i.e., surface) and a single sowing date, i.e., 1 November for wheat in the rabi season and zone-specific monsoon dependent sowing dates for rice in the kharif season. In reality, climate variables are not the only driver to cause major changes to yield. Next to the uncertainties in the climate data [
55,
57] a number of uncertainties in our results could be associated with the model limitations i.e., use of constant crop varieties, year-wise management decisions, and impacts of diseases and pests [
121,
122]. Model skills can be improved by validating simulated results with observations at local scale. Uncertainties in the yields statistics and observed climate data analysis (FAO and PBS crop yields data) can also be attributed to the expected human errors involved in reported low-quality agricultural data sets and use of station data to represent the whole district’s climate respectively [
122]. Assessments of crop yield responses to climate using models such as LPJmL, maybe further improved by implementing changing land-use scenarios, year wise changing sowing dates, and irrigation systems with changing irrigation efficiencies.
5. Conclusions
The objective of this study is to improve understanding of the impact of int(e)r(a)-annual climate variability on crop yields and crop water demand from irrigation in selected study sites of the IGB river basins in South Asia during the historical period 1981–2010.
Our results confirm the importance of climate-related assessments in crop yields and irrigation water demand at higher spatial (grid cell aggregated over study sites area) and temporal (crop phenological phases) scales. The results confirm that climate variables (i.e., temperature and precipitation) play a major role in crop development and growth. However, the degree of crop yield relationship strength with climate variables varies largely between seasons and among locations. Crop yields (i.e., wheat and rice) show very low sensitivity to climate variables (i.e., up to 4% to temperatures and up to 21% to precipitation) when assessed at the province and state level using observed yield and climatic data. However, crop yield showed a little higher sensitivity to temperature (up to 32%) and precipitation (up to 20%) variations at higher spatial scale i.e., districts level in Punjab Pakistan.
Simulated wheat and rice yields at 5 arc-min spatial resolution aggregated over selected study sites show that 27–72% variations in wheat and 17–55% variations in rice yields are linked with temperature variations in the rabi and kharif cropping seasons, respectively. In the absence of irrigation application, precipitation variations also play a major role, i.e., up to 39% variations in wheat yield and up to 75% variations in rice yield are directly linked with precipitation changes in the IGB river basins. Statistically significant and strong negative correlations between temperature and wheat yield indicate that wheat crop is quite vulnerable to heat stress. Kharif precipitation shows a statistically strong and positive relationship with rice yield production, indicating that a change in monsoon onset and uncertain climate extremes can impact the rice yield productivity.
Estimation of crop yield sensitivity to temperature and precipitation at high temporal scale, i.e., crop phase-specific, reveals that both wheat and rice crop yields are highly sensitive to reproductive phase temperatures (i.e., Pearson correlation of r from −0.33 to −0.86 for wheat and −0.33 to −0.71 for rice respectively). We conclude that wheat yields are most vulnerable to increasing winter temperatures in the reproductive phase. In the absence of irrigation application, both wheat and rice crop yields show mainly a significant positive relationship with crop phase-specific precipitation for all study sites with the strongest correlation, however with a large range, during the reproductive phase −0.12 to 0.75 for wheat and −0.18 to 0.77 for rice. Our analysis confirms that the crop yield sensitivity to climate variables depends on time and space specific climatic conditions.
Timing and quantity of irrigation water demand are also strongly associated with the variations in temperature and precipitation. We observed that irrigation water demand by both wheat and rice are generally positively correlated with temperature in both climate-sensitive crop phases with an exception during the reproductive phase of wheat where it shows a mixture (both positive and negative) of correlations for different locations. Whereas, crop phase-specific irrigation water demand by both crops show a negative relationship with precipitation i.e., under increased precipitation scenarios, decreased irrigation projections are expected. This study shows that crop phase specific climate variables play a major role in crop yield fluctuations within and between the years and also drive irrigation water demand in quantity and time. Therefore, improved knowledge on the shifts in irrigation water availability and demand based on local soil and climate conditions during sensitive crop growth phases and possible impacts on crop yields of rice and wheat in the IGB river basin will support adaptation strategies to cope with projected climate change and socio-economic scenarios.