CASA NPP predictions 2015 to 2022
The localized areas of Western Europe with the highest NPP levels predicted during the peak of summer growing seasons (Fig. 2) included: Cotswolds to North Wessex Downs in the UK; southeastern Belgium; woodlands north of Frankfurt Germany; Mounts of Cantal in the Massif Central of France; Vosges and Jura ranges of eastern France; northern Pyrenees mountains; eastern Italian Alps around Lago di Garda; Apennines range of western Italy; Karlovac and Lika-Senj Counties in northern Croatia. The southern Mediterranean coastal areas and islands of Spain, Italy, and Greece were predicted with the lowest NPP levels during the peak of the summer growing season; these regions display peak plant production in the cooler spring months of the year instead of the hot summer months.
Figure 2. Map of CASA predicted NPP for July 2018 in Western Europe
Averaged on a country-wide basis, total growing season NPP (g C m− 2) from 2015 to 2022 was shown to be highest (in a range of 200–300 g C m− 2) for Germany, France, Croatia, and the United Kingdom (Fig. 3). These four countries also had the narrowest range (geographically) of growing season NPP values over their total land coverage within any given year. Greece, Italy, Portugal, and Spain typically each had predicted growing season NPP from 2015 to 2022 in a lower range of 100–200 g C m− 2 and showed a relatively wider range of NPP values over their total land coverage within any given year.
Average growing season NPP declined markedly between 2021 to 2022 in Croatia, France, Germany, Italy, Portugal, and Spain (Fig. 3). Average plant production in Italy and Spain was predicted at relatively low levels during the growing seasons of 2016 and 2017 as well.
Figure 3. Average growing season NPP (g C m− 2) for selected Western European countries from 2015 to 2022 displayed with their Interquartile Range
Over the study period of 2015 to 2022, several notable high and low periods were identified for growing season NPP sums (in total grams C) for most countries in western Europe (Fig. 4). Across the summed land coverage of France, Greece, Italy, Portugal, and Spain, 2018 was the year with the highest terrestrial plant growth over the study period. Across the UK, Germany, and Croatia, 2017 and 2019 were the years with the highest summed NPP during the study period. For most of these eight selected countries, 2022 was the year predicted with the lowest summed plant growth, the exceptions being Germany and Greece, for which 2022 was the second lowest year of plant growth in the time series.
Figure 4. Comparison of summed growing season NPP (g C) in selected countries from 2015 to 2022.
Precipitation and temperature variations 2015 to 2022
Annual precipitation in most countries of Western Europe progressively declined from a high average rate of greater than 90 cm y− 1 in 2018 to a low average precipitation rate of less than 80 cm y− 1 in 2022 (Fig. 5a). The exception was the UK, which received its highest precipitation rate in 2020 during the study period, instead of in 2018. Croatia, France, Italy, Portugal, and Spain all received relatively low precipitation amounts in 2017 as well. Croatia, Germany, and Greece were the countries with the consistently narrowest range of variation in precipitation rates across their respective land cover area. The countries with the consistently widest range of geographic variation in precipitation rates were Spain, France, and Italy, particularly in 2022.
Annual average air temperatures increased slightly over the study period of 2015 to 2022 in Western Europe, mainly owning to a continental-wide warming trend from 2021 to 2022 (Fig. 5b). The countries with the consistently widest range of geographic variation in surface temperatures across their land area coverage were Greece and Italy.
Figure 5. Annual precipitation totals in cm y− 1 and annual average air temperatures (degrees Celsius) for selected Western European countries from 2015 to 2022 displayed with their Interquartile Range.
CASA NPP Response to Extreme Drought in 2022
The relative change (in percent) of NPP from 2021 to 2022 (Fig. 6) across sub-regions of Western Europe was most notable across the following geographic coverages:
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A pattern of increased (+ 20%) to decreased (-35%) growing season NPP from Ireland across to eastern England from 2021 to 2022, respectively.
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A pattern of decreased (-40%) to increased (+ 10%) growing season NPP from the western Loire Valley of France across to the Vosges from 2021 to 2022, respectively.
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A pattern of decreased (between − 25 and − 40%) growing season NPP in the Elbe River basin of eastern Germany from 2021 to 2022,
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A pattern of decreased (between − 25 and − 60%) growing season NPP across all of Spain, southern France, and northern Italy and Croatia from 2021 to 2022,
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A pattern of increased (between + 30 and + 45%) growing season NPP across the eastern Alps and in North Macedonia and in eastern Greece from 2021 to 2022.
Figure 6. Change (in percent) in the summed growing season NPP from 2021 to 2022 across Western Europe
CASA NPP at Landsat Resolution for Major River Basins
For a closer examination of drought impacts, three large river basins were selected that typified the pattern of decreased growing season NPP in excess of -50% from 2021 to 2022 in northern Spain, southern France, and northern Italy. These were the Ebro, Garrone, and Po River basins (Fig. 7). Land cover in these three river basins was predominantly croplands (33%) and open woodlands and shrublands (22%) (Buchhorn et al., 2020).
Figure 7. Location and extent of the Ebro, Garrone, and Po River basins in southern Europe
For all three of these large river basins combined, the major land cover classes within which growing season NPP was most strongly impacted by drought conditions 2022 (compared to the 2020 and 2021 growing seasons) were shrubland at -27%, cropland at -25%, and open forest at -21% (Table 1). The maximum predicted growing season NPP in 2022 was reduced by 30% in these land cover classes. The variability of growing season NPP within the three river basins combined also contracted by 11–13% in 2022 compared to the two previous growing seasons.
Table 1
Change in growing season NPP from 2020/21 to 2022 predicted by the CASA model for land cover classes (Buchhorn et al., 2020).in the combined Ebro, Garrone, and Po River basins in Spain, France, and Italy, respectively.
Land Cover Class | | Area (km2) | % Change 2020/21 to 2022 |
| | | | Maximum | Mean | Standard Deviation |
Shrubland | | | 12,958 | -30 | -27 | -13 |
Herbaceous vegetation | | 26,086 | -30 | -14 | -12 |
Cropland | | | 71,078 | -30 | -25 | -12 |
Urban | | | 8,982 | -30 | -21 | -11 |
Permanent water bodies | | 1,463 | -16 | -13 | -10 |
Herbaceous wetland | | 280 | -34 | -21 | -14 |
Closed forest, evergreen needle leaf | 11,719 | -19 | -13 | -5 |
Closed forest, deciduous broad leaf | 27,728 | -19 | -16 | -15 |
Closed forest, mixed | | 4,326 | -16 | -9 | 9 |
Closed forest, unknown type | 9,621 | -23 | -18 | -7 |
Open forest, evergreen needle leaf | 550 | -9 | -16 | 12 |
Open forest, deciduous broad leaf | 2,407 | -19 | -18 | -10 |
Open forest, mixed | | 1,490 | 0 | -18 | -7 |
Open forest, unknown type | | 34,627 | -30 | -21 | -11 |
The Ebro River in the northern Iberian Peninsula flows 930 km into the Mediterranean Sea, forming a delta in the Province of Tarragona in southern Catalonia. The Ebro drainage area of 85,550 km2 is the largest in Spain. Nonetheless, its mean annual flow has decreased by approximately 29% during the 20th century due to construction of dams and the increasing demands for irrigation of croplands (Cruzado et al., 2002). The main crops grown in the Ebro Valley include barley, wheat, fruit and olive orchards, vineyards, corn, and alfalfa (Lecina et al., 2010).
Localities within the Ebro River basin (example in Fig. 8) where the 2020/2021 growing season NPP was reduced by 50% or more by drought conditions in 2022 included:
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Aragón-Arga River sub-basins, within which the main river course is dammed near Esteríbar to supply water to the Pamplona metropolitan area;
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Lower Gallego and Cinca sub-basins, flowing into the Ebro downstream of the city of Zaragoza where the historical discharge has been reduced to 10% of the natural rate upstream by diversions (Burger et al, 2007);
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Lower Martin River sub-basins, flowing from the Iberian Range and regulated by the Cueva Foradada dam near the localities of Alcaine and Oliete.
Figure 8. CASA model NPP at 30-m Landsat pixel resolution predicted for the growing seasons of 2020 (left) and 2022 (right) for the Ebro River basin drainages of the Aragon River to its north. Location of blue star symbol is 42.23o N, 1.75o W near the city of Milagro.
The Garonne River in southwest France and northern Spain flows 602 km from the central Pyrenees and the Massif Central to the Gironde estuary at the port of Bordeaux. Its watershed drains 55,000 km2 to the Atlantic Ocean. The forested slopes of the Pyrenees make up 35% of the watershed area. The alluvial aquifer of the Garonne is considered a large regional reservoir and an important irrigation source for the agricultural activities in the Garonne Valley (Grusson et al., 2018). In addition to supplying over two million people with drinking water, more than 60% of the Garonne Valley is used for growing crops and livestock, with around 15% of this farmland in irrigated fields devoted mainly to corn, sunflower, and wheat (Martin et al., 2016).
Localities within the Garrone River basin (example in Fig. 9) where 2020/2021 growing season NPP was reduced by 50% or more by 2022 drought conditions included:
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Lower Aveyron and Le Tarn River sub-basins around the city of Albi. Principal crops grown in these valleys are maize, wheat, fruit trees, and alfalfa, much of which is irrigated (Rizzo et al., 2019);
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Lower Ger and Gimone River sub-basins around the city of Auch, flowing into the Garonne near Agen.
Figure 9. CASA model NPP at 30-m Landsat pixel resolution predicted for the growing seasons of 2020 (left) and 2022 (right) for the Garonne River basin drainages of Le Tarn River to the east of Albi. Location of blue star symbol is 43.94o N, 2.38o E near the city of Ambialet.
The Po River is Italy's longest, flowing 650 km through the northern Piedmont region and along borders of the Lombardy, Emilia-Romagna, and Veneto regions. The river's 46,000 km2 drainage basin forms the Po Valley, Italy's most important agricultural area, hosting a population of 17 million, a third of Italy's total population. Nonetheless, the valley’s surface runoff water is of little use for drinking and household uses, with flows being unreliable, often destructive in floods, and heavily polluted by sewage and fertilizers (UNEP, 2004). The Po River’s main uses are for hydro-electric power, irrigation, and industrial transport. The principal crops grown are wheat, maize, hay, barley, sugar beets, grapes, and rice.
Localities within the Po River basin (example in Fig. 10) where 2020/2021 growing season NPP was reduced by 50% or more by 2022 drought conditions included:
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Lower Tanaro River sub-basins north of the city of Genova. Principal crops grown in these lowlands around the city of Alessandria are maize and wine grapes;
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Lower Taro and Parma River sub-basins around the city of Fidenza. Principal crops grown in these lowlands are wheat, tomatoes, and alfalfa as forage for dairy cattle;
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Lower Reno River sub-basins southeast of the city of Bologna. The principal crop grown is maize, consuming 30% of irrigation water supplied in the Reno River valleys.
Figure 10. CASA model NPP at 30-m Landsat pixel resolution predicted for the growing seasons of 2020 (left) and 2022 (right) for the Po River basin drainages north of Parma. Location of blue star symbol is 44.99o N, 10.41o E near the city of Casalmaggiore.