Irrigation water demand of selected agricultural crops in Germany between 1902 and 2010

STOTEN-20336; No of Pages 16 Science of the Total Environment xxx (2016) xxx–xxx

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Irrigation water demand of selected agricultural crops in Germany between 1902 and 2010 Katrin Drastig a,⁎, Annette Prochnow a,b, Judy Libra a, Hagen Koch c, Susanne Rolinski c a b c

Leibniz-Institute for Agricultural Engineering Potsdam-Bornim, Max-Eyth-Allee 100, 14469 Potsdam, Germany Humboldt-University of Berlin, Faculty of Life Sciences, Chair Utilization Strategies for Bioresources, Hinter der Reinhardtstr, 8–18, 10115 Berlin, Germany Potsdam Institute for Climate Impact Research (PIK) Telegrafenberg, 14412 Potsdam, Germany

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• IWD of selected agricultural crops in Germany was determined between 1902 and 2010. • Changes in climatic conditions did not significantly affect the IWD. • Changes in the cropping pattern and cultivated area strikingly affected the IWD.

a r t i c l e

i n f o

Article history: Received 14 May 2016 Received in revised form 25 June 2016 Accepted 26 June 2016 Available online xxxx Editor: D. Barcelo Keywords: Irrigation water demand Irrigation trend AgroHyd Farmmodel

a b s t r a c t Irrigation water demand (IWD) is increasing worldwide, including in regions such as Germany that are characterized with low precipitation levels, yet grow water-demanding crops such as sugar beets, potatoes, and vegetables. This study aimed to calculate and analyze the spatial and temporal changes in the IWD of four crops—spring barley, oat, winter wheat, and potato—between 1902 and 2010 in Germany by using the modeling software AgroHyd Farmmodel. Climatic conditions in Germany continued to change over the investigation period, with an increase in temperature of 0.01 K/yr and an increase in precipitation of 1 mm/yr. Nevertheless, no significant increasing or decreasing trend in IWD was noted in the analysis. The IWD for the investigated crops in the area of the current “Federal Republic of Germany” over the 109 years was 112 mm/yr, varying between 100 and 127 mm/yr. Changes in cropping pattern and cultivated area over the last century caused large differences in the IWD calculated for each administrative district. The mean annual IWD of over the study period (which was divided into 4 parts) varied between 13,455 Mm3/yr in the earliest period (1902–1919) and 4717 Mm3/yr in the latest period (1990–2010).

Abbreviations: IWD, irrigation water demand; SD, standard deviation; Tact, actual crop transpiration; ET0, reference evapotranspiration; ETc, potential crop evapotranspiration; AWC, available water capacity; Kcb, basal crop coefficient; p: evapotranspiration depletion factor; Zr: effective rooting depth; LAI: leaf area index. ⁎ Corresponding author. E-mail address: [email protected] (K. Drastig).

http://dx.doi.org/10.1016/j.scitotenv.2016.06.206 0048-9697/© 2016 Published by Elsevier B.V.

Please cite this article as: Drastig, K., et al., Irrigation water demand of selected agricultural crops in Germany between 1902 and 2010, Sci Total Environ (2016), http://dx.doi.org/10.1016/j.scitotenv.2016.06.206

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Policy and management measures to adapt to climate change are currently being debated in Germany. The presented results suggest that the effects of the choice of crops (in this case, changes in cropping pattern in the German nation states) had a stronger influence on regional water resources than those of climate variability. Thus, the influence of climate change on water resources is relativized which brings an important input into the debate. © 2016 Published by Elsevier B.V.

1. Introduction Water conservation is an essential component of sustainable land management. Globally, water use for agriculture until 2010 has increased substantially by 75% compared to that in 1960 and by 400% compared to that in 1900 (Shiklomanov, 2000). About 18% of croplands worldwide, or about 2% of the total land surface, are irrigated and meet 40% of the global food demand (UNCSD, 1997). In Germany, 373,000 ha were used for irrigation in 2009 (FAOSTAT, 2015), accounting for 2.2% of cropland and 1% of total land surface in Germany. About 70% of all water from rivers and aquifers worldwide is used for agriculture (Siebert et al., 2013). Studies have focused on the implications of agricultural water and land-use at different scales (Biewald et al., 2014; Bonsch et al., 2015; Steidl et al., 2015a,b); however, further studies are needed to estimate the pressure of irrigation on the available water resources, irrigation water requirement, and irrigation water withdrawal (Frenken and Gillet, 2012). An important region for irrigation is the Mediterranean region (Egypt, Turkey, Syria, Spain, Morocco, Italy, France, Greece, Tunisia, Algeria, Libya, Israel …) where despite the semiarid conditions prevailing almost the whole year an intensive farming takes place, being highly specialized and varied in the kinds of crops raised. Cereals account for nearly the half of the total irrigated area (Daccache et al., 2011). Irrigation supports crop diversification, helps assure yield level and quality and helps to stabilize food supplies as precipitation in this region is subject to high inter-annual and seasonal variability. In semiarid zones aridity, water scarcity, and economic development are affecting the irrigation water demand in various ways. Furthermore the ongoing debate on climate change impacts on the water cycle has raised concerns regarding future water availability in Central-European regions (Weatherhead and Knox, 2000). Wriedt et al. (2009) used the crop growth model EPIC and simulated the irrigation water demand (IWD) in Europe for 8 years between 1995 and 2002. The irrigation requirements were simulated based on the spatial distribution of irrigated areas, climatic conditions, and crops. The simulated net irrigation requirements ranged from 53 mm/yr in Denmark to 1120 mm/yr in Spain, indicating the estimated net volumetric irrigation requirements of 107 Mm3/yr and 35,919 Mm3/yr, respectively. Regional and global climate simulations performed using different definitions of drought with medium confidence levels indicate an increase in the duration and intensity of droughts in Central and Southern Europe and the Mediterranean, further north as far as the UK (IPCC, 2014). Even in regions where summer precipitation is expected to increase, soil moisture and hydrological droughts might become more severe because of the increasing evapotranspiration (Wong et al., 2011). Asymmetry was found for these trends if the study period was split into two subperiods. For precipitation, all Europe-average indices of wet extremes increased, although the spatial coherence of the trends was low. Klein Tank and Können (2003) analyzed the trends of daily temperature and precipitation extremes in Europe since 1946 and found a warming episode in 1946–1999. Schönwiese and Janoschitz (2008) reported an increasing trend of the annual mean temperature in Germany during 1901–2000 of 1 K and an increasing trend for the annual precipitation sum of overall 9%. During 1971–2000, the increase was 15%. Trömel and Schönwiese (2008) found that summer precipitation in Germany from 1901 to 2000 showed negative trends in the most of the climate stations; the time series did not detect negative precipitation trends in the southern

part of Germany. Furthermore, the distribution shifted to higher annual precipitation sums in the southern part of Germany in winter and in summer. In Germany, rainfall for crops is usually sufficient. Hydrologically, Germany is associated with a humid climate with an average annual precipitation (± standard deviation, SD) of 773 ± 97 mm/yr (DWD, 2015). Precipitation decreases from the west to east. Irrigation is supplementary and used to optimize production in dry springs and summers, especially when water stress occurs during a sensitive crop growth stage. About 6% of farms in Germany have the equipment to irrigate fields that are located in particularly dry or intensively farmed regions, such as fruit and vegetable production areas (Federal Statistical Office and the Statistical Offices of the Länder, 2011). In the current German federal states, 8.5% of the agricultural production area is equipped for irrigation in Lower Saxony; 2.8%, in RhinelandPalatinate; 2%, in Hesse; 1.9%, in North Rhine-Westphalia; and 1.6%, in Brandenburg (Federal Statistical Office and the Statistical Offices of the Länder, 2011). In the light sandy soils in the North and East German regions, the water retention capacity is low, and yearly precipitation is not sufficient to supply optimally the cereals, potatoes, and vegetables. Local sites could lose their productivity if they are subjected to more frequent and long-lasting droughts without irrigation (Drastig et al., 2011). In southern Germany, clayey and silty soils with high water-holding capacity might need additional water supply at lower precipitation and for crops that are more sensitive to water stress, such as sugar beets, potatoes, and vegetables. The German Strategy for Adaptation to Climate Change recommends further development of water-saving agricultural management practices and irrigation technologies to deal with current and future challenges. The main yield-limiting factor probably is the growing shortage of water (Deutscher Bundestag, 2008). We hypothesized that the observed change of inter-annual distribution of precipitation from summer to winter and an increasing temperature might have changed the IWD in Germany. This study aimed to analyze the spatial and temporal variations in the IWD in Germany between 1902 and 2010, and to determine the trends for the IWD over these 109 years for four selected crops, i.e., spring barley, oat, winter wheat, and potato. After the study area was characterized, the climate data, soil data, plant-specific parameters, and local cultivation practices were obtained. The IWD of the crops was modeled using the AgroHyd Farmmodel (Drastig et al., 2012), and trend analysis was performed for the climate variables, IWD, cultivated area, and production and yield for each crop. 2. Material and methods 2.1. Administrative districts, cultivation area, annual production, and yield The study area was Germany, including its borders between 1902 and 2010. During these 109 years, the political borders, land area, as well as the available data changed substantially. Land areas for the modeling process were defined by deriving 153 districts from the administrative districts of historical maps. In the timeline showing the concurrent changes in political borders, area, and administrative districts over the 109 years, various constellations of these 153 districts led to the formation of the different German nation states (Fig. 1). The analysis of these states began in 1902 with the territory of the “German Empire,” which existed from 1871 to 1918 and included 38 administrative districts. In this study, 31 administrative districts in “Weimar

Please cite this article as: Drastig, K., et al., Irrigation water demand of selected agricultural crops in Germany between 1902 and 2010, Sci Total Environ (2016), http://dx.doi.org/10.1016/j.scitotenv.2016.06.206

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Fig. 1. Timeline of changes in the German nation states, land area, and administrative districts over the last century and the availability of statistical data for the administrative districts during the four time periods.

Republic” and 28 administrative districts in “National Socialism” were considered. The data for this study were not available after the beginning of World War 2 from 1941 until the foundation of the two German national states in 1949. After 1950, the 11 administrative districts of the territories of the Western “Federal Republic of Germany” and 15 administrative districts of the Eastern “German Democratic Republic” were considered as a basis. After the German reunification, 16 administrative districts of the current “Federal Republic of Germany” were analyzed between 1993 and 2010 (Fig. 2). For the periods 1902–1919, 1920– 1940, 1950–1989, and 1990–2010 (Fig. 1), all administrative districts with their respective land areas were defined for the investigation to achieve homogenization of the time series of available data and for modeling results. The historical maps were obtained from the Historical Geographical Information System of the German states and territories in the 19th century (HGIS Germany; Kunz and Dietze, 2007). For the series until 1945, these maps were based on the data of the Max Planck Institute for Demographic Research (MPIDR) and the Chair for Geodesy and Geoinformatics, University of Rostock (CGG; MPIDR and CGG, 2011)

and Hubatsch and Klein (1975). For the series from 1945 to 2008, the maps of HGIS Germany were based on the data of MPIDR and CGG (2011) and Federal Agency for Cartography and Geodesy (BKG, 2011). For the analyses for 2009 and 2010, maps from BKG (2011) were used. Annual statistical data on cultivated area [Mha], production [Mt], and yield [t/ha] were used from various sources. The values for the “German Empire,” “Weimar Republic,” and “National Socialism” were obtained from Imperial Statistical Office (ISO, 2013) and Statistisches Reichsamt (StRA, 2013). The values for the “Federal Republic of Germany” (1952–1990) were obtained from Destatis (2013a). The values for the German Democratic Republic (1950–1990) were obtained from Staatliche Zentralverwaltung für Statistik (SZS, 2013) and those for the “Federal Republic of Germany” (1991–2005) were obtained from Destatis (2013b). The values for the last investigated time series (2006–2010) were obtained from Destatis (2013c). The annual data on cultivated area [Mha] and yield [t/ha] were available between 1901 and 2010. The annual data on production [Mt] were available between 1933 and 2003. For the years during which data were not available, i.e., between 1902 and 1932 as well as 2003 and 2010, the

Fig. 2. Districts included during the different time periods for the calculation of volumetric IWD and for trend analyses—for data only available district wise (yield, area, production, and IWD [Mm3]). Colors denote the different districts.

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production values were calculated by multiplying the area data with the yield data for the respective years. For the yield [t/ha], area [ha], and IWD [mm] data sets available within the borders of the “Federal Republic of Germany,” performing regression analyses covering the entire period between 1902 and 2010 was possible. For the data sets of yield, area, production, and volumetric IWD [Mm3] that were available only district wise, analyses were performed for the four time periods for the four respective external borders (Figs. 1 and 2).

capacity (AWC) of the soil in the root zone between two thresholds. When the AWC in the root zone dropped below 70% during the irrigation period, water was added. The upper threshold for irrigation water application was 85% AWC. For the evaluation of the modeling results, simulated values were compared to measured values for actual evapotranspiration based on lysimeter measurements, to modeled IWD values, recommended IWD values in handbooks, and finally to applied irrigation water values that were reported.

2.2. Calculation of the IWD by using the AgroHyd Farmmodel 2.2.2. Choice of crops and crop parameters The four crops, spring barley (Hordeum vulgare L.), oat (Avena sativa L.), winter wheat (Triticum aestivum L.), and potato (Solanum tuberosum), were selected for investigation because of their relevance for irrigation and data availability on area, yield and, partly, production. Data for rye, spring barley, oat, wheat, potato, and grassland were available from 1901 until 2010. At present, irrigation in Germany is primarily applied for vegetables, fruits, and vineyards, as well as specialist products (e.g., hops) and, secondarily, for crops such as potato and sugar beet. Cereals, primarily winter barley, are irrigated in some German states, in addition to feed crops (Michel and Sourell, 2014; Roth et al., 1995). Only 9%–14% of the irrigation water utilized in Germany was used for the three grains investigated in this study (Michel and Sourell, 2014). Winter wheat was selected because it is the most important cereal crop in Germany. Nearly 20% of the current agricultural area in Germany is cultivated with winter wheat (Federal Statistical Office and the Statistical Offices of the Länder, 2011). Spring barley cultivation is important in Germany because of its use in breweries and malt houses, which causes its comparably higher economic value and potential for increased net benefit from irrigation. Rye was not considered in the study because it is a resilient crop that is usually not irrigated in Germany, but is an important crop with nearly 10% of the agricultural area in Germany planted with cereals for the production of rye. German farmers earn almost every seventh Euro from cereal growing; in 2013, the production value amounted to around 7.5 billion Euros (BMEL, 2014).The plant-specific parameters and periods shown in Table 1 were used for the parameterization of the AgroHyd Farmmodel. The Kcb, evapotranspiration depletion factor, and height were obtained from Allen et al. (1998). Effective rooting deep reported by Allen et al. (1998) and Kutschera et al. (2009) was used. Leaf area index was obtained from Scurlock et al. (2001; winter wheat); Särekanno et al. (2010; potato), and Liu et al. (2010; spring barley and oat).

2.2.1. Modeling process The IWD over the 109 years was calculated using the modeling software AgroHyd Farmmodel (Drastig et al., 2012). This model allows the systemic quantification of water flows at different scales—from the farm scale by using individual farm operating data up to the regional scale on the basis of a daily time step. Local climate data, obtained from climate stations operated by the German Meteorological Services (DWD), crop data from various sources, and soil data obtained from FAO-UNESCO Soil Map of the World (FAO, 1971–1981; Fischer et al., 2009) were combined in a modeling unit for detailed calculation of the local hydrologic processes: evaporation from soil, transpiration, rainfall interception loss, and percolation. In this study, 4159 modeling units were obtained from overlaying the 153 districts with polygons of soil data. Water flows were calculated for each modeling unit on a daily basis for 109 years. The algorithm for calculating the actual crop transpiration (Tact) in the AgroHyd Farmmodel is based on the FAO 56 dual crop coefficient method (Allen et al., 1998). This requires calculation of the (1) reference evapotranspiration (ET0), (2) potential crop evapotranspiration (ETc), (3) actual evapotranspiration (ETact), and actual transpiration (Tact). The following steps are implemented in the AgroHyd Farmmodel: ET0 of a grass reference surface is calculated using the FAO Penman–Monteith equation by using regional climate data (Allen et al., 1998). Subsequently, it is adjusted for individual crops by using plant-specific parameters, e.g., the plant-specific basal crop coefficient (Kcb), to calculate Tc. A daily soil water balance obtained using a simple tipping bucket approach is combined with regional soil and precipitation data to determine the water stress coefficient that reduces Tc to Tact (Allen et al., 1998). Thus, the calculation of Tact incorporates the effect of daily water stress owing to water-limited conditions by linking the data sets on plants and soil characteristics. In addition, the deep percolation of water through the soil, total available soil water in the root zone, root zone depletion, evaporation, and ET0 were calculated according to Allen et al. (1998). In this model, rainfall interception was calculated as described by von Hoyningen-Huene (1983) and Braden (1985). A detailed description of the model is provided in Drastig et al. (2012) and Prochnow et al. (2012). Within a predetermined irrigation period (Table 1), irrigation water was modeled to be supplied to the crops to maintain the available water

2.2.3. Soil parameters The AWC in mm/m of soil units of the Harmonized World Soil Database v1.2 (Fischer et al., 2008) was used. The map is a 30 arc-second raster database that combines existing regional and national soil information with that contained within the 1:5,000,000 FAO-UNESCO Soil Map of the World (FAO, 1971–1981; Fischer et al., 2009).

Table 1 Plant-specific parameters for crop-related modeling (p: evapotranspiration depletion factor, Zr: effective rooting depth, Kcb: basal crop coefficient, LAI: leaf area index). Crop

Spring barley

Phase

Initial

Start day End day p (−) Zr (m) Height (m) Kcb (−) LAI (m2 m−2) Vegetation period Irrigation period Fallow

1 82 81 126 0.5 0.5 0.3–1.4 1 0.15 1.1 0.2–1.8 15/03–15/08 15/05–05/07 16/08–14/03

Mid-season

Oat

Winter wheat

Late season

Initial

127 152 0.6

1 82 81 126 0.5 0.5 0.3–0.55 1 0.15 1 0.2–1.8 15/03–15/08 20/5–10/07 16/08–14/03

0.2

Mid-season

Late season

Initial

127 152 0.55

1 226 155 295 0.55 0.55 0.3–0.9 1 0.15 1.1 0.2–2.7 18/09–31/07 10/5–10/07 01/08–17/09

0.15

Mid-season

Potato Late season

Initial

296 316 0.9

1 67 66 116 0.7 0.35 0.35–0.7 0.6 0.15 1.1 0.2–3.4 15/04–15/09 15/5–05/07 16/09–14/04

0.15

Mid-season

Late season 117 152 0.4

0.65

Please cite this article as: Drastig, K., et al., Irrigation water demand of selected agricultural crops in Germany between 1902 and 2010, Sci Total Environ (2016), http://dx.doi.org/10.1016/j.scitotenv.2016.06.206

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2.2.4. Climate data Daily data from the 368 climate and precipitation stations in the network operated by the DWD corrected by the Potsdam Institute for Climate Impact Research (PIK; Österle, 2001; Österle et al., 2006) were available for 1902–2010. For each modeling unit, data on daily temperature, precipitation, relative air humidity, sunshine duration, and wind speed from the next closest stations were used without interpolating. The climate data from 1902 to 2010 were used for the territory of the “Federal Republic of Germany.” For administrative districts outside this area, the climate variables were extrapolated. 2.3. Trend analysis method Two types of trend analyses were used to identify the trends in climatic and hydrological variables, including transpiration, yield, IWD, and area. For the normally distributed values, the parametric least squares linear regression test for trends (Hess et al., 2001) with a significance level (α) of 5% was applied. Values were calculated for the period 1902–2010 in the region of the “Federal Republic of Germany.” The non-parametric Mann–Kendall's test was applied for trend detection (Maidment, 1993) by using a significance level (α) of 5% for the values not showing normal distribution. This was necessary for the IWD values in [m3/yr]. These values were calculated for the four selected time periods, i.e., 1902–1919, 1920–1940, 1950–1989, and 1990– 2010, for the districts in the study area of the German nation states (Fig. 2). 3. Results 3.1. Climate and cropping pattern and associated trends 3.1.1. Climate The mean values and trends in climate variables over the 109 years are summarized in Table 2 for the territory of the current “Federal Republic of Germany.” The measured annual precipitation of 764 mm/yr was higher than the modeled reference evapotranspiration of 621 mm/yr, indicating that Germany, on average, does not have a water deficit. The trends over time suggested that precipitation, ET0, and minimum and maximum temperatures increased annually. The increase in precipitation over the past century by 109 mm was higher than that in the ET0 by 44 mm. Between 1902 and 2010, the maximum and minimum annual temperatures increased by 1.20 °C and 0.98 °C, respectively. The driest year in the current territory was 1934, followed by 1921, both with a mean annual precipitation of b580 mm/yr. The wettest years were 2002 and 1999, with a mean annual precipitation above 980 mm/yr. At present, the driest German federal states are Berlin and Brandenburg located in the eastern part of Germany, with precipitation of b 600 mm/yr. The wettest federal states are found in southwestern Germany: Bayern, Baden-Württemberg, and North-Rhine Westphalia, with N 940 mm/yr precipitation.

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3.1.2. Cultivation area and cropping pattern Shifts in the cultivation area (Table 3) over the 109-yr period resulted from the changes in the cropping pattern over time as well as the size of the study area. At the beginning of the investigation, oat and potato were the most important crops that accounted for cultivation areas between 3 and 4 Mha, which were about twice the size for those of winter wheat or spring barley. However, by the end of the investigation, the cultivation areas of oat and potato were reduced by 90%. The cultivation area of barley also decreased over the 109 years to half its initial value, whereas the importance of winter wheat increased. At present, winter wheat is widely and extensively cultivated, followed by barley, grain maize, and oil seeds (Destatis, 2010a). Winter wheat cultivation expanded even though its total agricultural area used decreased. The trends in cultivation area were analyzed for the four time periods. In 1902–1919, the trend was significant only for spring barley. In 1920–1940, significance was noted for all crops except for oat, and from 1950 onward (1950–1989 and 1990–2010), the trends were significant for all the crops (Table 3). 3.1.3. Production During 1902–1919 and 1920–1940, the largest production rate among the four crops was noted for potato (40–41 Mt; Table 4). Production decreased by 72% to 11 Mt in 1990–2010, which was lower than the winter wheat production of 22 Mt. In contrast, winter wheat production expanded considerably over the four periods—increasing by 428% in relation to the first period and showing the most significant increasing trend. This corresponded to the trend for the increase in cultivated area described above. When the trends for winter wheat were compared among the four time periods, the largest increase was found in the last period (1990–2010), with a mean annual increase of 0.7 Mt. The two previous periods also showed significant increasing trends for winter wheat. The greatest significant trend was found for potato production. In 1920–1940, an annual increase of 1.179 Mt was found for potato. 3.1.4. Yields The decreases in cultivated area over the 109 years were offset by the increase in yields for all the four crops (Table 5). Wheat showed the largest increase in yield of 352%, increasing from 2.3 t/ha/yr to 8.1 t/ha/yr over the four periods. The trend analysis showed significant yield increases for all the crops after 1919 except for spring barley and oat in the fourth period. The highest trends were noted for potato production, especially in the last period, with 0.582 t/ha/yr. The annual potato production decreased only by one-fourth, although the production area decreased to 10% because of the increase in potato yield. 3.2. Modeling results 3.2.1. Water flows The local hydrologic processes were calculated using the AgroHyd Farmmodel for the 4159 modeling units. The regional mean values for potato crop between 1902 and 2010 were different between the

Table 2 Annual climate values (mean ± standard deviation, SD) and annual climate trend (significance, *p ≤ 0.05) for the period 1902–2010 and the area of the current “Federal Republic of Germany” of 357,340 km2.

Precipitation [Mm3] Precipitation [mm] Reference evapotranspiration [Mm3] Reference evapotranspiration [mm] Humidity [%] Maximum annual temperature [°C] Minimum annual temperature [°C] Sunshine duration [h] Wind speed [m/s]

Annual values (mean ± SD)

Annual trend

Trend between 1902 and 2010

252,431 ± 31,828 764 ± 96 205,336 ± 11,179 621 ± 34 79 ± 1.1 12.5 ± 0.8 4.5 ± 0.7 4.4 ± 0.4 2.6 ± 0.1

+317* +1.00* +136* +0.40* −0.003 +0.011* +0.009* −0.00001 +0.0003

+34,553* +109* +14,824* +43.6* −0.327 +1.199* +0.981* −0.00109 +0.0327

Please cite this article as: Drastig, K., et al., Irrigation water demand of selected agricultural crops in Germany between 1902 and 2010, Sci Total Environ (2016), http://dx.doi.org/10.1016/j.scitotenv.2016.06.206

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Table 3 Mean cultivated area [Mha] with standard deviation (SD) and annual trends [Mha/a] of the cultivated area covering the data of the districts within the study area (significance, *p ≤ 0.05 detected using Mann–Kendall test).

Total area

1902–1919

1920–1940

1950–1989

1990–2010

10.27

8.18

5.36

3.74

Annual values (mean ±

Spring barley Oat Winter wheat

Annual trend

Annual values (mean ±

SD) 1.47 ± 0.1

−0.012*

1.41 ± 0.2

0.018*

1.32 ± 0.5

3.95 ± 0.4 1.77 ± 0.2

0.002 −0.010

2.78 ± 0.6 1.62 ± 0.3

−0.024 0.039*

1.02 ± 0.2 1.80 ± 0.4

SD)

Annual values (mean ±

Annual trend

vegetation period and fallow period for each year (Fig. 3). The data for precipitation, IWD, actual evapotranspiration, deep percolation, and soil water balance are shown in the figure. Precipitation amounts decreased similarly during the 211-day fallow and 153-day vegetation periods. The increase in precipitation of 109 mm over the past century was noted mainly in the fallow period. For example, in 2002, half of the precipitation of 1011 mm/yr decreased during the vegetation period from April 15 to September 15. Therefore, the IWD was small (63.3 mm) for that year. The soil storage depletion during the vegetation period was made up during the fallow period. This resulted in significantly deeper percolation in the fallow period, whereas evaporation and soil water storage remained approximately at the same level (Table 6). The water flows during the potato vegetation period showed less variability. Although precipitation in the spring and summer did not change significantly, small increases in actual evapotranspiration and deep percolation were significant.

3.2.2. Irrigation water demand The mean values of the annual IWD over the 109 years for individual crops were similar, ranging from about 100 mm/yr for spring barley and potato to 120 mm/yr for oat and 127 mm/yr for wheat (Table 7a). The mean value for all four crops was 112 mm/yr. For a better representation, they can be compared to the mean annual ET0 of the German nation states, showing that the IWD amounts to 16% (spring barley), 19% (oat), 20% (wheat), and 16% (potato) of the ET0 values. These high mean values for the IWD [mm] for the four crops resulted in high values for the volume of irrigation water [m3] required for their cultivation in all the districts in the study area (Table 7b). Since the cultivation area decreased over the four investigation periods, the volume of IWD also decreased, although the mean IWD for all four crops remained relatively constant over the 109 years (Fig. 4). The highest value for IWD was noted in 1976 and the lowest in 1987. Other peaks in IWD corresponded with a low precipitation amount, e.g., in 1934, 1947, 1959, 1964, and 1973. IWD for the four crops varied widely from 4717 m3/yr to 13,455 m3/ yr, with the minimum in 1987 and maximum in 1915 (Fig. 4). The IWD decreased by 65% during 1990–2010 compared to that in 1902–1919.

SD)

Annual trend

Annual values (mean ± SD)

Annual trend

0.027*

0.68 ± 0.2

−0.024*

−0.021* 0.032*

0.26 ± 0.1 2.49 ± 0.4

−0.012* 0.041*

The mean annual irrigation water demand for the four crops was calculated for the four time periods. The years 1976, 1989, 1915, 1917, and 1959 had the highest mean annual IWD (N160 mm) with potato cultivation covering the cropland of the current “Federal Republic of Germany” (Fig. 4).

3.2.3. Development of the crop-specific IWD The IWD consistently followed the order—winter wheat N oat N spring barley N potato—for the crop production over the four investigation periods (Fig. 5, Table 9). The modeled volume of the crop-specific IWD [m3/yr] for the four crops over the four time periods shifted with changes in the cultivation area (Fig. 6). The volume of the irrigation water required for winter wheat became the highest during 1990–2010, whereas that for the other three crops reached the lowest value. The annual volume increased for winter wheat from 2622 Mm3/yr during 1902–1919 to 3668 Mm3/yr during 1990–2010. This 40% increase in the volume of required irrigation water was the only increase noted over the four periods among the four crops. Irrigation water required for spring barley decreased by 63% from 1755 Mm3/yr (1902–1919) to 652 Mm3/yr (1990–2010), whereas that for oat and potato showed remarkable decrease: 94% from 5290 Mm3/yr (1902–1919) to 311 Mm3/yr (1990– 2010) for oat and 92% from 3788 Mm3/yr (1902–1919) to 286 Mm3/ yr (1990–2010) for potato.

3.2.4. Crop-specific trends No significant trends in the IWD [mm/yr] were detected for the four investigated crops over the four time periods, individually or as mean values (Table 8a). Analysis of the total volume of required irrigation water [Mm3/yr] for all the four crops did not show a significant trend for the first three periods; however, in the last period (1990–2010), the increasing trend of 109 Mm3/yr for all the crops was significant (Table 8b). Corresponding mainly to the changes in cultivation area, the modeled IWD for Germany showed several significant changes in the mean annual volume of required irrigation water for the investigated crops (Table 8b).

Table 4 Mean production [Mt] with standard deviation (SD) and annual trends of production [Mt/yr]] including the data for the districts within the study area shown in Table 1 (significance, *p ≤ 0.05 detected using Mann–Kendall test). 1902–1919 Annual values (mean ± SD) Spring barley Oat Winter wheat Potato

1920–1940 Annual trend

Annual values (mean ± SD)

1950–1989 Annual trend

Annual values (mean ± SD)

1990–2010 Annual trend

Annual values (mean ± SD)

Annual trend

2.78 ± 0.6

−0.054*

2.74 ± 0.5

0.083*

4.76 ± 2.3

0.175*

3.01 ± 0.7

−0.060*

7.12 ± 1.7 4.08 ± 0.8

−0.136 −0.033

5.31 ± 1.3 4.11 ± 1.2

0.050 0.154*

3.26 ± 0.5 9.02 ± 4.4

−0.011 0.378*

1.20 ± 0.3 21.52 ± 5.9

−0.052* 0.708*

40.54 ± 8.9

−0.345

40.29 ± 8.5

1.179*

26.79 ± 8.4

−0.768*

10.85 ± 1.7

0.088

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Table 5 Mean annual yield [t/ha] with standard deviation (SD) and annual trends of yield [t/ha/yr] including the data for the districts within the study area (significance, *p ≤ 0.05 detected using Mann–Kendall test). 1902–1919 Total area [Mha]

54.81 Annual values (mean ±

Spring barley Oat Winter wheat Potato

1920–1940

1950–1989

48.14 Annual values (mean ±

SD)

Annual trend

1.8 ± 0.3 1.8 ± 0.3 2.3 ± 0.3 12.5 ± 1.8

−0.024 −0.015 −0.008 −0.09

1990–2010

35.47 Annual values (mean ±

SD)

Annual trend

1.9 ± 0.3 1.9 ± 0.3 2.4 ± 0.3 14.6 ± 2.4

0.038* 0.035* 0.039* 0.365*

35.73 Annual values (mean ±

SD)

Annual trend

SD)

Annual trend

3.3 ± 0.7 3.2 ± 0.67 4.5 ± 1.1 21.6 ± 3.4

0.060* 0.057* 0.091* 0.233*

4.7 ± 0.4 4.6 ± 0.5 8.1 ± 0.7 34.8 ± 4.8

0.003 −0.009 0.070* 0.582*

Yield of cereals in tons of dry matter with 14% humidity; yield of potato in tons of fresh matter.

3.3. Spatial variability of the IWD between 1902 and 2010 during climatic extreme years In addition to temporal trends, we investigated the spatial distribution of IWD for extremely dry years 1934 (Fig. 7) and 1976 (Fig. 8) and for the extremely wet year 2002 (Fig. 9). The year 1934 was associated with the lowest annual precipitation of 572 mm/yr, with only 309 mm recorded during the vegetation period of potato. The mean potential and actual evapotranspiration in the vegetation period were similar (490 mm and 433 mm, respectively). The high IWD of 147 mm occurred because of the combination of high ET0 and low precipitation (Fig. 7). Regions associated with the highest IWD are the north-eastern German lowlands within the federal state of

Brandenburg and in the area in and around the Upper Rhine Valley and Neckar valley within the current federal states of Hesse and RhinelandPalatinate (Fig. 7). These regions also showed the highest actual evapotranspiration. The lowest actual evapotranspiration was found in northwestern Germany in the current federal state of Lower Saxony. The precipitation values were the highest in south Germany, in the state Bavaria. The highest values for actual evapotranspiration and IWD formed a band reaching from the southwest to the north-east of Germany. It covered mainly the biogeographical region Continental, which is situated between the Alpine and the Atlantic biogeographical region, in Germany. Furthermore, the detected band might be assigned to the agroclimatic zone Atlantic Central located in Germany, following the proposition of Iglesias et al. (2012; Fig. 7). This area is characterized by an IWD of N150 mm/yr.

Fig. 3. Mean water flows for potato in the vegetation (left) and fallow (right) periods over the 109 years: mean inflows (precipitation, irrigation), mean outflows (actual evapotranspiration, deep percolation), and soil water balance in the area of the current “Federal Republic of Germany.”

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Table 6 Annual water flow values [mm/yr] (mean ± standard deviation, SD) and annual trend [mm/yr] (significance, *p ≤ 0.05) over the 109 years for the area of the current “Federal Republic of Germany” of 354,872 km2 for the fallow and vegetation periods of a potato crop. Annual values (mean ± SD)

Annual trend

370 ± 67 188 ± 15 178 ± 57

+0.74* +0.14* +0.59*

Vegetation period (15.04–15.09) Precipitation 350 ± 56 Actual evapotranspiration 496 ± 35 Deep percolation 74 ± 26

+0.25 +0.34* +0.19*

Fallow period (16.09–14.04) Precipitation Actual evapotranspiration Deep percolation

Table 7 Mean annual irrigation water demand (a) in [mm/yr] and (b) in [Mm3/yr] and standard deviation (SD) for the area of the current “Federal Republic of Germany” for all the districts in the study area. (a) Irrigation water demand [mm] demand [mm] [mm]

1902–2010

Mean value of the four crops Spring barley Oat Winter wheat Potato

112 ± 31 101 ± 31 120 ± 30 127 ± 38 100 ± 30

Summarized crops Spring barley Oat Winter wheat Potato

4. Discussion 4.1. Climate trends

The extreme dry year 1976, with a similar low mean annual precipitation of 598 mm/yr and only 260 mm in the vegetation period, produced the highest mean IWD of 178 mm/yr during the investigation period (Fig. 8). The IWD was higher in this year than in 1934 because the reference evapotranspiration was higher (517 mm/yr). The mean potential evapotranspiration and actual evapotranspiration differed by nearly 100 mm/yr—the high potential evapotranspiration could not always be satisfied with irrigation water (not shown here). This was the largest difference between reference and actual evapotranspiration of the three extreme years, i.e., 1934, 1976, and 2002. The actual evapotranspiration was the lowest in northern Germany, increasing toward the south. The highest precipitation values were also found in the southern state Bavaria. A band of IWD could also be noted stretching across the area of the 24 administrative districts during 1950–1989 with the “German Democratic Republic” and the “Federal Republic of Germany.” The values for IWD in this zone from the southwest to the north-east of Germany were considerably higher than the mean IWD of 178 mm. The north-eastern German lowlands and the area around the Upper Rhine Valley and Neckar are associated with the highest actual evapotranspiration and IWD, reaching up to 270 mm. The wettest year in the investigated period was 2002, with the highest mean annual precipitation of 1011 mm. The precipitation amount within the vegetation period was also the highest (474 mm). This resulted in a small difference between the actual and ET0. As in the other years, the highest precipitation values were noted in the southern state Bavaria and in the western federal state of North Rhine-Westphalia (Fig. 9). The highest actual evapotranspiration was found around the Upper Rhine Valley and Neckar valley. Further, a similar band of IWD was noted across Germany, following the biogeographical region Continental and the agroclimatic zone Atlantic Central (Fig. 9). The highest IWD was associated with the north-eastern German lowlands and the area around the Upper Rhine Valley and Neckar Valley

(b) Irrigation water demand [Mm3/yr]

and, even for the wet year 2002, the average values of N90 mm were reached.

Precipitation, as well as temperature and ET0, increased in Germany over the past century. The mean annual precipitation increased approximately by 1 mm/yr, whereas the annual minimum and maximum temperatures increased (0.009 °C and 0.011 °C per year, respectively) with the corresponding increases in the potential evapotranspiration of 0.4 mm/yr. These findings are consistent with those of Schönwiese and Janoschitz (2008) who analyzed the climate-related trends in Germany from 1901 to 2000 and found similar increases over the 100 years: + 0.9 °C for the average annual temperature and + 9% for the annual precipitation. Wind speed and humidity were the two variables that showed no significant trend between 1902 and 2010. Between 1902 and 2010, the mean annual values of precipitation exceeded those of ET0. Since the positive trend in annual precipitation was 2.3-times larger than that of ET0, the climate could have assumed to become more humid over the investigated years. However, comparison of precipitation amounts in the fallow vs. vegetation periods showed that the 109-mm increase in precipitation dropped mainly in the winter periods; hence, only winter crops benefitted in the vegetation period. Trömel and Schönwiese (2008) found that precipitation remarkably increases mainly in winter, especially in December, whereas, it decreases in summer, mainly in August. Therefore, even if the annual precipitation increases, an intra-annual shift might lead to an increased IWD. 4.2. Cropping areas The changing food and feed demands over the century as well as the major improvements in crop yields caused large changes in the cultivated areas of individual crops. The total cultivated area of all four crops was reduced by 64% owing to the changing borders and consumption patterns over the 109 years, whereas the yield increased by 2.5- to 3.5-fold. At the beginning of the last century, oat and potato were the most important crops, having cultivated areas that were about twice the size of those of winter wheat or spring barley. Oat was important as a food crop as well as horse feed until the end of the 19th century because large numbers of horses were used for transportation and agricultural purposes. Mechanization reduced the importance of oat as a feed crop and replaced it with potato for human consumption (KörberGrohne, 1995). By the end of the investigation period, the cultivation areas of both oat and potato were about ten-fold smaller than that of winter wheat, a reduction by about 90%. The cultivated area of spring barley also decreased to less than one-third of the area of winter wheat, losing half its initial value, whereas the importance of winter wheat increased. Wheat became the most important breadstuff in Germany over the last century (Körber-Grohne, 1995). At present, the cultivated area of winter wheat covers twice the land area of the other three crops combined. While winter wheat production is distributed between the German federal states of Bavaria, Lower Saxony, and SaxonyAnhalt, potato production is regionally more concentrated, with the highest area found in Lower Saxony. 4.3. Irrigation water demand

1902–1919 13,455 ± 3468

1920–1940

1950–1989

1990–2010

10,614 ± 3225 5820 ± 2245 4717 ± 1371

1755 ± 468 1555 ± 536 5290 ± 1439 3637 ± 1101 2622 ± 650 2321.86 ± 860 3788 ± 1018 3101 ± 1047

1273 ± 722 652 ± 203 1197 ± 470 311 ± 89 2118 ± 1042 3668 ± 1168 1232 ± 623 286 ± 74

4.3.1. Spatial variability The investigation of spatial variability of the IWD showed a band of higher demand stretching from the south-west to the north-east of Germany within the Continental biogeographical region. Differences in IWD ranged from 265 mm/yr for wheat in southern Germany to 183 mm/yr in the north-east. The current German federal states

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Fig. 4. Time series of the mean annual modeled IWD covering the data for all districts within the study area for potato cultivation. The mean values for the four crops were averaged for IWD [mm/yr] and summed up for volume [Mm3/yr]. Lines show the total cultivation area for each time period (red) and the mean irrigation values (light blue) for the four crops. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Rhineland-Palatinate, Hesse, and Brandenburg located in the region Upper Rhine Valley and Neckar Valley and in the regions of north-eastern German lowlands are the states threatened most by insufficient precipitation. The current state Bavaria receives higher amount of mean annual precipitation that results in sufficient rainwater for the crops. Differences among regions were found over the 109 years. The band of higher IWD from the south-west to the north-east of Germany is consistent with the finding of Riediger et al. (2016). The authors investigated two areas each in the north-western (Diepholz and Uelzen) and north-eastern German regions (Fläming and Oder-Spree). In the latter, the annual IWD demand, e.g., for potato, was approximately 175 mm higher than that in the north-western German regions (between 25 and 60 mm during 1991–2010). This applies to all crops investigated in the study. Riediger et al. (2016) found that the water required for crop irrigation in Germany is likely to increase with considerable regional variation. 4.3.2. Trends In this study, the intra-annual shift combined with increased annual precipitation caused the IWD to remain approximately constant. This applies for the mean value of the four selected crops (112 ± SD 31 mm/yr) as well as for each individual crop. Riediger et al. (2016)

also found no trend during 1991–2010. They modeled regional variability of irrigation requirements due to climate changes in Northern Germany. Although climate variability can cause large changes in IWD, important drivers for change can be of anthropogenic origin; changes in land use, i.e., in cropping pattern and cultivated area, have greater effect on water requirements. This is in line with the findings of Riediger et al. (2016) who found no significant trend for the IWD within the investigated regions of north-western and north-eastern Germany for their reference period 1991–2010. In contrast, in the Haihe River Basin, China, the total net irrigation water requirement was found to decrease over the past 50 years mainly because of the reduction in the duration of the crop growth period (Yuan et al., 2014). In our study, the growth period length was assumed to remain stable. A smaller irrigation water demand during the reduced growth period in Germany could be attributed to the increased temperature found between 1902 and 2010. The IWD did not change over time in response to the trends in temperature and precipitation. However, the analysis showed that variation in IWD among the years can be extremely large, from b50 mm/yr in 1923 to N 250 mm/yr in 1976. This could cause a fourfold increase in the IWD in certain regions, thereby leading to the exploitation of water resources over individual years and conflicts among users.

Fig. 5. Time series of simulated annual irrigation water demand (IWD) [mm/yr] for spring barley, oat, winter wheat, and potato within the area of the current “Federal Republic of Germany.” Values for spring barley were only slightly lower than those for potato.

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Fig. 6. Time series of simulated annual volume of required irrigation water for spring barley, oat, winter wheat, and potato based on the cultivation areas of the districts within the study area.

Considering the simulated volume, the crop-specific IWD [m3/yr] varied with the changes in the cultivation area. The volume of required irrigation water for winter wheat was the highest during 1990–2010, whereas it was the lowest for the other three crops. In this study, for the four investigated crops, a mean annual IWD of 4717 Mm3 was simulated for an area of 35.73 Mha for the period 1990–2010. Aus der Beek et al. (2010) used the framework WaterGAP (Alcamo et al., 2003) and found a lower IWD of 585 Mm3 for 18 different crop types compared to that found in the present study. Schaldach et al. (2012) used the model WaterGAP3 and reported a mean annual IWD of about 1917 Mm3 for 13 different crop types for the year 2000 in Germany by taking into account a real and considerably smaller irrigated area of 0.267 Mha compared to that used in the present study. 4.3.3. Influencing factors Cropping area, which was assumed to be irrigated completely, had the largest influence on the amount of IWD in Mm3. The irrigation of an area planted with a respective crop depends on the viability of irrigation, which is controlled by economic condition alone—it depends on additional costs and additional profits of irrigation. The additional costs are controlled by the irrigation technique used and water prices. Additional profits depend on higher yields of the respective crops and Table 8 Annual trends in irrigation water demand (a) in mm/yr for the area of the current “Federal Republic of Germany” detected using least squares linear regression parametric test and (b) in Mm3/yr for all the districts in the study area (significance, *p ≤ 0.05 detected using Mann–Kendall test). (a) Trend in irrigation water demand [mm/yr]

1902–2010

Mean for all 4 crops Spring barley Oat Winter wheat Potato

−0.05 −0.02 0.01 −0.05 −0.04

b) Trend in irrigation water demand [Mm3/yr]

1902–1919 1920–1940 1950–1989 1990–2010

Mean for all 4 crops Spring barley Oat Winter wheat Potato

−80.53 −17.50 −49.20 −4.36 −5.70

109.41 34.30 * −9.61 45.64* 55.56

6.41 32.78* −22.42 * 36.26 * −39.10 *

109.27* −10.88 −10.18* 125.03* 2.49

on higher prices for the products. Climate change might lead to higher yields, but can be accompanied by increasing water deficits that might influence the water prices. Furthermore, climate change scenarios might have a significant negative impact on the value of farmland. Water availability will strongly capitalize into farmland values, as shown by Schlenker et al. (2007) for California, USA. El Chami et al. (2015) showed that supplemental irrigation of wheat in the UK, which was previously considered as uneconomic, can be justified for farmers in the East of England who have unused irrigation equipment and summer water. The authors considered climate data from 1961 to 2011 and wheat prices from 2007 to 2011. The expected increase in world wheat prices and their impact on climate change are likely to increase the financial benefits of irrigation.

4.4. Comparison of simulated and reported values for individual flows 4.4.1. Simulated actual evapotranspiration—comparison with values from lysimeter measurements Single water flow components were further analyzed by comparing model results to measured values for actual evapotranspiration based on lysimeter measurements in the Weimar region in Thuringia (Central Germany) for the crops spring barley, winter wheat, and potato (Roth et al., 2005) and in Hesse (Central Germany) for oat (Ernstberger, 1987), as well as by using further published values for IWD (Hanke, 1986; LUFA, 2010; Michel and Sourell, 2014). In Thuringia, irrigation water was applied to the lysimeter to compensate 30% of the water demand based on the climatic water balance (Roth et al., 2005). Two measured values per year from the lysimeter measurements for wheat were compared to the model results (Fig. 10). Actual evapotranspiration values agreed well with those for the observed years of 1983, 1987, and 1992, whereas the IWD modeling results were higher than the actually applied amount. This can be explained mainly by the different prerequisites of irrigation management, which was restricted to 70% of AWC for the modeled values and compensated 30% of the water demand for the lysimeter experiments. Furthermore, the higher simulated IWD was derived with lower precipitation values than that observed at the lysimeter stations. From a visual inspection, the values of the simulated actual evapotranspiration between the years 1983–1995 and all investigated crops agreed well with the mean observed data from lysimeter measurements

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Fig. 7. Precipitation [mm], simulated actual evapotranspiration (ETact) [mm], and irrigation water demand (IWD) [mm] for potato cultivation in the vegetation period of the extreme dry year 1934. Administrative districts during 1920–1940.

(Fig. 11). Comparable values for the IWD for different crops were not available. 4.4.2. Simulated irrigation water demand—comparison with recommended values The mean values for IWD simulated over the period 1902–2010 ranged between 100 and 127 mm/yr for all the four crops. These model results were compared to those from selected literature sources on IWD. IWD depends on plant and soil properties, as well as on climate and the accepted level of uncertainty for plant water supply (i.e., irrigation control); it can vary widely across regions. Comparison with recommended values for irrigation scheduling, e.g., from the handbooks for Central Germany (Hanke, 1986), showed that the model results are in the upper range (spring barley, winter wheat, and potato) or above (oat) the recommended IWD (Table 9). The higher value for oat was probably attributed to the higher range of certainty of water supply selected in this study (70%–80% of AWC vs. 60%–80% used in Hanke, 1986). Another general comparison to the results from an eight-year simulation of irrigation requirements in Central and Northern Europe (United Kingdom, Belgium, Netherlands, Luxemburg, Germany, Denmark,

and Sweden) for crop categories used in the European Farm Structure Survey also showed good agreement (Wriedt et al., 2009). The value of IWD for cereals, sunflower, sugar beet, rape, and potato remained below 200 mm/yr, as was found in this study. The irrigation scheduling within the different modeling approaches and within the handbooks used in this study for a comparison is intended to maintain soil moisture to a level that ensures the quality and yield stability of crops. The IWD results of the reported studies are attributed to the different methods used for calculating water balances and different prerequisites. The following approaches were based on the climatic water balance: • LUFA (2010): Additional water amount should be applied for a maximum of 130% compensation of the climatic water balance depending on the respective crops without calculating the soil water balance for overhead irrigation. Roth et al. (2005) calculated IWD during a cropspecific irrigation period and by using crop-specific values, e.g., 30% for wheat. WaterGAP: Within the framework WaterGAP (Alcamo et al., 2003 cited by Aus der Beek et al., 2010 and Wriedt et al., 2009), compensation of 100% of climatic water balance was considered to calculate water deficits over the vegetation period, which are

Fig. 8. Precipitation [mm], simulated actual evapotranspiration (ETact) [mm], and irrigation water demand (IWD) [mm] for potato cultivation in the vegetation period of the extreme dry year 1976. Administrative districts during 1950–1989.

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Fig. 9. Precipitation [mm], simulated actual evapotranspiration (ETact) [mm], and irrigation water demand (IWD) [mm] for potato cultivation in the vegetation period of the extreme wet year 2002. Administrative districts during 1990–2010.

accumulated to estimate the annual IWD by considering the limited water availability. Further, efficiency of irrigation was considered.

Other approaches based on climatic water balance and soil water balance: • In this study, IWD was calculated during an irrigation period for a respective crop. After the climatic water balance was calculated, the soil water balance allowed the calculation of IWD. The prerequisite was to maintain the AWC of the respective soil in the root zone between the two thresholds of 70% and 85% AWC. Effects of irrigation efficiency were not considered, and water availability was set to be unlimited. • Riediger et al. (2014, 2016) used a trigger for irrigation; if available, the soil water dropped below 20% AWC. The effects of irrigation

efficiency were not considered, and water availability was set to be unlimited. • Michel and Sourell (2014) assessed the IWD values for 80% years, the 80th percentile, with secure water supply. Hanke (1986) used the IWD values for 60%–80% years with secure water supply to calculate the IWD. Irrigation water should be applied sufficiently for percent of years [%] in consideration of AWC and climatic water balance for different crops on different soils for the two approaches. The values were obtained from nomograms (Roth et al., 1981).

As expected, the highest IWD values were calculated for the high (N80%) compensation of the water balance (LUFA, 2010) and for the approaches considering unlimited water availability (this study). The lowest IWD values were calculated for the low (b 20%) compensation of the

Fig. 10. Comparison between actual evapotranspiration values, other water fluxes, and soil water balance derived from measurements by using two lysimeters in 1983, 1987, and 1992 (Roth et al., 2005) and those simulated using the AgroHyd Farmmodel for wheat cultivated in Weimar (Thuringia).

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Fig. 11. Comparison between actual evapotranspiration (ETact) values for the years in the period 1983–1995 derived from 17 lysimeter measurements and those simulated using the AgroHyd Farmmodel.

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Fig. 12. Comparison between the reported applied irrigation water [Mm3] and IWD values [Mm3] simulated using the AgroHyd Farmmodel for the years 2002 and 2009 for the reported irrigated areas. The points represent values for 15 current German federal states excluding Bremen, Berlin, and Hamburg. The year 2002 was rather wet with precipitation level of N1000 mm/yr.

water balance (Riediger et al., 2014, 2016) and for the approaches considering limited water availability (Alcamo et al., 2003). 4.5. Future implications 4.4.3. Modeled IWD—comparison with reported values Data on the national level of the actual amount of irrigation water used annually for crops investigated in the study period are limited. Data from two national statistical surveys in Germany for the years 2002 (Destatis, 2004) and 2009 (Destatis, 2010b) were used to compare the volume of reported irrigation water used [Mm3/yr] to the simulated values for the four crops (Fig. 12). The IWD was calculated using the reported irrigated areas and the AgroHyd Farmmodel by using plant-specific input parameters (Table 1), soil parameters, and site-specific parameters of the long-term modeling. Comparison of the two years showed that both the reported and simulated values in 2009, a year with average precipitation, were considerably higher than those in the wet year 2002 (Fig. 12). In both the years, the amount of irrigation water used was only 60% of the IWD calculated using the model. A significant coefficient of determination (R2) with a level of p b 0.05 was obtained using a value of 0.99. However, the simulated IWD and the reported amount of annual irrigation water used do not necessarily have to be similar. The reported amount of irrigation water used could have been considerably higher or not enough—it does not represent an optimal amount. There could have been, for example, an exceptionally high simulated daily IWD, that was not accounted for during the calculation of the mean annual simulated IWD. The values based on the recent irrigation infrastructure might not be sufficient to meet the peak irrigation needs on a daily basis. The result could have been considerably lower values than the simulated ones. Table 9 Evaluation of the AgroHyd Farmmodel for calculating the IWD [mm/yr] with standard deviation and range of recommended IWD [mm/yr] reported, simulated, and calculated values from the literature for Central Germany (mean ± SD).

Spring barley Oat Winter wheat Potato a b c

Simulated for 1902–2010 in this study (N70% AWC)

Literature

101 ± 31

20–105a simulated

120 ± 30 127 ± 38

65–100b simulated 15–130a calculated

100 ± 30

55–150a simulated, 132c calculated

Michel and Sourell (2014), 80% security of water supply. Hanke (1986), 60%–80% security of water supply. LUFA (2010) 1994–2008, 100% of water balance.

For the past 102 years, the significant climatic changes did not cause significant trends in irrigation water demand in Germany. Nevertheless, with proceeding climate changes, the irrigation water demand is expected to increase. Germany is the largest agricultural producer of milk, pig meat, and rapeseed in the 27-strong European Union (BMELV, 2009) and, in international agricultural trade, Germany is the world's third largest exporter (BMEL, 2015). Although supplemental irrigation for crops is still rare in Germany, the tendency is that larger areas are equipped for irrigation in order to ensure reliable product quality and quantity (Deutscher Bundestag, 2008). In the German states within the band of high IWD, agriculture is becoming challenging because of water availability (Drastig et al., 2012; Schütze et al., 2016). Conflicts over the limited surface water resources, sinking groundwater tables, as well as decreasing water quality are expected to rise (Steidl et al., 2015a,b). The impact of climate change on cropping patterns and growing seasons will intensify, leading to an expected increase in net irrigation requirements in the North of Germany from 72 mm to 90 mm in 2020 (Döll, 2002). For German wheat production, a substantial expansion of irrigation into the rain-fed areas will be required to maintain the current yield (Tanaka et al., 2015). The temporal pattern of a potential change in IWD is an important factor with regard to the functioning of the irrigation infrastructure, because a pronounced increase in the daily use of IWD changes the requirements for the infrastructure than an evenly distributed increase in IWD (Shahid, 2011). The tails of temperature and precipitation distributions, but not the means, are the dominant climatic determinants during irrigation adaptation (Hegri et al., 2005). An increase in IWD is also expected for other countries having a humid climate. For irrigated potato production in England, future potential yields, without restrictions in water or fertilizers, are expected to increase by 13%–16% (Daccache et al., 2011). Future average irrigation needs, assuming unconstrained water availability, were predicted to increase by 14%–30%. However, the recent irrigation infrastructure was assessed to fail with regard to the satisfaction/fulfillment of future peak irrigation needs in nearly 50% of the years investigated. Conversely, the rising atmospheric CO2 concentrations might counteract an increase in the IWD. CO2 effects were found to alter the direction of IWD change in various regions producing grain maize and sugar beet toward a reduction in water demand (Zhao et al., 2015). Because of the effect of increased CO2 on stomatal aperture, the water use of potato crops will be reduced by 11% (Jaggard et al., 2010).

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In our study, although climate change did not significantly affect the IWD, the changing cultivation area of crops had a strong impact on IWD. In the future, the irrigated area might expand due to the increase in the cultivation of bioenergy crops. As farmers become fuel suppliers for power plants, it becomes more important to achieve stable yields even in the dry years (Gutzler et al., 2015). Chaturvedi et al. (2013) investigated the climate mitigation policy implications for global irrigation water demand. They found that the largest increases in IWD occurred during emission mitigation policy scenarios where fossil fuel emissions, but not land use change emissions are priced. These scenarios were primarily driven by the rapid expansion in bioenergy production. Various measures to adapt to an expected local and temporal future water scarcity in Germany and to reduce the increase in IWD can be proposed. Since the main yield-limiting factor probably is the growing shortage of water, the German Strategy for Adaptation to Climate Change (Deutscher Bundestag, 2008) recommends further development of water-saving agricultural management practices and irrigation technologies. Measures to improve rainwater use efficiency in the field should be applied (Drastig et al., 2012), such as humus conservation management and direct seeding to improve soil moisture retention, supporting deeper rooting of crops, breeding drought-tolerant crops, and tactically choosing crops and crop rotations that enable plants to use the water resources optimally. The cultivation of winter crops that benefit from the intra-annual shift of the precipitation pattern in favor of winter precipitation could be expanded. Weed control to avoid competition with crops for water, optimal nutrient supply of young plants to promote rapid development of ground cover, accompanied by reduction of unproductive evaporation and sufficient potassium supply needs to be ensured. In addition to agricultural management practices aiming at a more efficient use of rainwater, irrigation might be a beneficial adaptation mechanism to the changes even in climate extremes in the future (Troy et al., 2015), thereby essentially decoupling the crop yields from climate. However, while the irrigation water demand increases under climate change, water availability might decrease; hence, a strategic regional water management is required in order to possibly limit irrigation. An increase in water use for irrigation can be associated with decreases in the water cycle, especially in the annual streamflow. This might lead to the alteration of the entire hydrologic regime. Intensive irrigation through alluvial aquifer withdrawal modified the climatic feedback and altered streamflow response to precipitation in an agricultural watershed in the Southern Great Plains, USA (Dale et al., 2015). Along with groundwater withdrawal, anthropogenic-induced factors and activities contributed to nearly half of the observed variability of annual streamflow. The consequences of the altered hydrologic regime might have cascading trophic effects that might extend beyond the direct influence of the river itself. This is especially important in semiarid zones where the economic development of these regions needs to take into account the sustainability of the water resource use to minimize the impact on the water sources while dealing with water scarcity and climate change effects (Sapriza-Azuri et al., 2015). For instance, in the case of the Guadiana Basin in Spain, the effectiveness of various adaptation measures for irrigated agriculture is ranked the highest for those related to private farming (new crop varieties and modern irrigation technologies), whereas public-funded hard measures (reservoirs) were ranked the lowest and public-soft measures (insurance) are ranked the middle (Varela-Ortega et al., 2016). Under environmental aspects, regional water demand restrictions might be needed (Henriques et al., 2008). With regard to our study, the cultivation of water-demanding crops such as sugar beets, potatoes, and vegetables within the band of higher IWD stretching from the south-west to the north-east of Germany should be planned wisely under consideration of available groundwater and surface water for irrigation. Furthermore, the IWD can be reduced by applying water-saving irrigation technologies and irrigation scheduling.

5. Conclusions Despite significant increases in temperature and precipitation between 1902 and 2010, the irrigation water demand in Germany did not change significantly. The increase in precipitation was counterbalanced by an intra-annual shift from the vegetation period to the winter months. Large regional differences with some places having high irrigation water demand forming a band from southwest to northeast Germany reflect the Continental biogeographical region and Atlantic Central agroclimatic zone. In contrast to climate change, the area cultivated with crops potentially relevant for irrigation has a large influence on the irrigation water demand. Thus, for adaptation strategies to the expected local and temporal water scarcity in the future, cropping pattern is of high importance. Economic incentives and more regional coordination of cropping pattern and associated water rights could act as a solution. In addition to the changes in climate and cultivated crops, the development of irrigation costs and product prices determine the need for and viability of irrigation, and hence the irrigation water demand. Acknowledgments This work was supported by the Leibniz Competition (formerly SAW Procedure) within the Leibniz Association, grant number SAW-2011ATB-5. The authors gratefully acknowledge the support from Andreas Kunz from the Leibniz Institute of European History, Section World History. The authors declare that they have no conflicts of interest. The authors gratefully acknowledge the support from three anonymous reviewers. References Alcamo, J.M., Döll, P., Henrichs, T., Kaspar, F., Lehner, B., Rösch, T., Siebert, S., 2003. Development and testing of the WaterGAP2 global model of water use and availability. Hydrol. Sci. J. 48, 317–337. http://dx.doi.org/10.1623/hysj.48.3.317.45290. Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements vol. 56 (Rome, Italy. http://www.fao.org/ docrep/X0490E/X0490E00.htm/ (accessed 21.09.15)). Aus der Beek, T., Flörke, M., Lapola, D.M., Schaldach, R., Voß, F., Teichert, E., 2010. Modelling historical and current irrigation water demand on the continental scale: Europe. Adv. Geosci. 27, 79–85. http://dx.doi.org/10.5194/adgeo-27-79-2010. Biewald, A., Rolinski, S., Lotze-Campen, H., Schmitz, C., Dietrich, J.P., 2014. Valuing the impact of trade on local blue water. Ecol. Econ. 101, 43–53. http://dx.doi.org/10.1016/j. ecolecon.2014.02.003. BKG, 2011. Bundesamt für Kartographie und Geodäsie, Germany VG 2500 Verwaltungsgebiete (Ebenen) 1:2.500.000. Version January 2009, Frankfurt Am Main. BMEL, 2014. Understanding Farming - Facts and Figures about German Farming. Federal Ministry of Food and Agriculture (http://www.bmel.de/SharedDocs/Downloads/EN/ Publications/UnderstandingFarming.pdf?__blob=publicationFile (accessed 06.06.16)). BMEL, 2015. Understanding Agricultural Exports, Facts and Background Information. Federal Ministry of Food and Agriculture (http://www.bmel.de/SharedDocs/Downloads/ EN/Publications/Understandingagriculturalexports.pdf?__blob=publicationFile/ (accessed 14.03.16)). BMELV, 2009. German Agriculture Facts and Figures. http://www.bmel.de/SharedDocs/ Downloads/EN/Publications/GermanAgriculture.pdf?__blob=publicationFile (accessed 14.03.16). Bonsch, M., Popp, A., Biewald, A., Rolinski, S., Schmitz, C., Weindl, I., Stevanovic, M., Högner, K., Heinke, J., Ostberg, S., Dietrich, J.P., 2015. Environmental flow provision: implications for agricultural water and land-use at the global scale. Glob. Environ. Chang. 30. Braden, H., 1985. Ein Energiehaushalts- und Verdunstungsmodell fuer Wasser und Stoffhaushaltsuntersuchungen landwirtschaftlich genutzter Einzugsgebiete. Mitt. Dtsch. Bodenkdl. Ges. 42, 294–299. Chaturvedi, V., Hejazi, M., Edmonds, J., Clarke, L., Kyle, P., Davies, E., Wise, M., 2013. Climate mitigation policy implications for global irrigation water demand. Mitig. Adapt. Strateg. Glob. Chang. 20, 389–407. http://dx.doi.org/10.1007/s11027-0139497-4. Daccache, A., Weatherhead, E.K., Stalham, M.A., Knox, J.W., 2011. Impacts of climate change on irrigated potato production in a humid climate. Agric. For. Meteorol. 151, 1641–1653. http://dx.doi.org/10.1016/j.agrformet.2011.06.018. Dale, J., Zou, C.B., Andrews, W.J., Long, J.M., Liang, Y., Qiao, L., 2015. Climate, water use, and land surface transformation in an irrigation intensive watershed—streamflow responses from 1950 through 2010. Agric. Water Manag. 160, 144–152. http://dx.doi. org/10.1016/j.agwat.2015.07.007.

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