Water Needs of Sweet Cherry Trees in the Light of Predicted Climate Warming in the Bydgoszcz Region, Poland

Abstract

The Bydgoszcz region (Poland) is located in an area with a very high demand for supplementary irrigation during the vegetation period of plants. The projected global warming will bring a rise in the water needs of crops, and thus a further increase in irrigation needs. The goal of the study was an attempt to estimate the water needs of sweet cherry trees in 2021–2050 (forecast period) in the region of Bydgoszcz. The years 1981–2010 were adopted as the reference period. The water needs of sweet cherry trees were calculated on the basis of air temperature using the Treder method, in which water needs are equated with the potential evapotranspiration of a given fruit tree species. It was found that in the growing season of the forecast period, the relative diversity of sweet cherries’ water needs was relatively small (7%). The highest variability of monthly water needs was in April, May, and June. The seasonal water needs amounted to 573 mm, with very high monthly water needs noted in July (139 mm) and August (134 mm). A significant trend of the time variability of water needs was calculated only in August. During this month, it is predicted that the water needs will rise by 5 mm in each subsequent decade. These results will be helpful in the design of sweet cherry irrigation treatments.

1. Introduction

Orchards located in the Bydgoszcz region cover over 40% of the area of horticultural crops in this region. Unfortunately, their production is often relatively low, which is caused by a large, over 30% share of light soils and very diverse meteorological conditions in the growing season in this region. Therefore, to ensure high yields, orchard plants should be provided with additional irrigation supplementing precipitation in the amount of 100 to 200 mm of water [1,2,3,4,5].

The water requirements of different species of fruit trees vary greatly [4,6,7,8]. Cherries, peaches, pears, and walnuts belong to a group of plants with intermediate water requirements [4,6,7,9]. However, it is sometimes believed that cherry and sweet cherry trees have lower water needs than other species of fruit trees, such as plum, apple, or pear [8]. According to Treder and Pacholak [8], sweet cherry trees, like cherry trees, require an annual amount of precipitation in the range of 500 to 600 mm. However, as stated by Kemmer and Schulz [7], the annual rainfall for cherry trees, depending on the average air temperature in the period from May to September, ranges from 460 mm (for a temperature of 14 °C) to 620 mm (for a temperature of 17 °C). According to Ostrowski [10], in the climatic conditions of Poland, wherever the annual rainfall is lower than 700 mm, irrigation improves the conditions for the growth of fruit plants, especially on light soils with low water capacity. Rzekanowski [5] reported that the optimal annual precipitation for cherries, determined according to Kemmer and Schulz’s assumptions, ranged from 473 to 539 mm in a five-year study period while the rainfall deficits for cherry trees in the two driest years ranged from 187 to 192 mm. This corresponds well with the seasonal irrigation standards for sweet cherries and cherries, which, as per Słowik [6], range from 120 to 140 mm, and, according to Dzieżyc [7], are 160 mm.

Taking into account the assumptions formulated by Press, it is estimated that in the growing season, i.e., from April to September, the water needs of cherry trees grown on medium-light soil at average air temperature, amount to 360 mm, ranging from 325 mm at low temperature up to 400 mm at high temperature [11]. Cherry orchards on light sandy soils are characterized by greater water needs, which, for low, medium, and high-temperatures, amount to 390, 432, and 480 mm, respectively. On the other hand, on heavy, compact soils (loams and clays), the water needs of cherry trees are 260, 288, and 320 mm, respectively, for low, medium, and high temperatures [11,12].

In light of the climate change scenario considered to be the most probable, it is predicted that at the end of the 21st century, the increase in global temperature will amount to 2–4 °C. One can also expect an increase in winter precipitation and a decrease in summer precipitation [13,14].

Scenarios of temperature and precipitation changes in the summer period, i.e., from April to August, developed for Poland, differ significantly from each other [15,16]. All models predict a rise in air temperature, but only some of them predict a rise in rainfall. There are also scenarios that forecast a reduction in rainfall [15,16,17,18]. In view of the above, it is estimated that the predicted climatic changes may significantly increase the water needs of plants [15,16].

A calculation of the water needs of sweet cherry trees in the years 2021–2050 in the Bydgoszcz region was the purpose of this study. Estimations were made for the growing season (from 1 April to 30 September) on the basis of the expected increase in air temperature. The Bydgoszcz region is situated in north-central Poland, in the Kuyavian-Pomeranian Province (Figure 1); this is an area with a very high requirement for supplementary irrigation in the growing season [2,3,19,20,21,22].

2. Materials and Methods

In this study, the water needs of cherry trees (Prunus avium L.) were estimated in accordance with the climate change scenario for Poland based on the Special Report on Emission Scenarios (SRES): A1B [17,18]. The projected average monthly temperature values for the Bydgoszcz region in the years 2021–2050, which constituted the forecast period, were used in the calculations. As the reference, a thirty-year period from 1981 to 2010, was adopted. The average monthly air temperature in the years 1981–2010, recorded in Bydgoszcz (φ = 53°06′ N, λ = 18°01′ E and 80 m a.s.l.) at the Institute of Technology and Life Sciences meteorological station, were used in the calculations.

The main methodology of crop water needs is based on the most common FAO approach, where potential evapotranspiration is estimated from weather-based reference evapotranspiration (ETo) and the crop coefficient (kc) [23]. To calculate the water needs of sweet cherries, the Treder method [24], described in detail by Rolbiecki [25], was applied. In the Treder method, the water needs of sweet cherries were identified with the potential evapotranspiration (ETp) (called by Treder, the crop evapotranspiration) of a given fruit tree species. The ETp of sweet cherries was estimated using Formula (1):

$$\mathrm{ETp}=\mathrm{kc}\times \mathrm{ETo},$$

(1)

where ETp = potential evapotranspiration, i.e., crop evapotranspiration in conditions without water deficits in soil (mm); kc = crop coefficient for sweet cherries according to Treder (

Table 1. Values of the empirical coefficient and crop coefficient for sweet cherries according to Treder [24,25].

Month	Empirical Coefficient	Crop Coefficient
April	0.28	0.50
May	0.21	0.75
June	0.19	1.10
July	0.18	1.20
August	0.17	1.20
September	0.16	1.15

); and ETo = reference evapotranspiration (mm), calculated by the Formula (2):

$$\mathrm{ETo}=\mathrm{n}\times \mathsf{\alpha }\times \mathrm{t},$$

(2)

where ETo = reference evapotranspiration (mm); n = number of days in a month; α = empirical coefficient determined by Treder (Table 1); and t = average monthly air temperature (°C).

Statistical analysis was performed on the obtained results. The maximum, minimum, mean, and median, as well as the standard deviation and variability coefficient, were also calculated. In addition, by means of linear regression analysis, with the determination of correlation and determination coefficients, the possible trends of changes in sweet cherries’ water needs in both compared periods were determined. With a sample size of n = 30, the significance of correlation coefficients was determined for p = 0.05. Therefore, according to the confidence interval, the correlation coefficient was significant for rα ≥ 0.362 [26].

The number of rainfall deficits for sweet cherry trees in the normal (N_50%), medium dry (N_25%) as well as very dry (N_10%) years was calculated by the Ostromęcki method [11,27,28,29]. The following Formula (3) was used to calculate the precipitation deficit:

$$\mathrm{Np}\%=\mathrm{Ap}\%\times \mathrm{ETp}-\mathrm{Bp}\%\times \mathrm{P},$$

(3)

where Np% = precipitation deficit at the probability occurrence p% (mm period⁻¹); Ap% and Bp% = numerical factors characterizing the variability of evapotranspiration and precipitation for a given meteorological station; ETp = multi-year average evapotranspiration in the analyzed period (mm period⁻¹); and P = multi-year average precipitation in the analyzed period (mm period⁻¹).

3. Results

The values of standard deviation calculated in this study show the diversity of the monthly sums of water needs of sweet cherries. In the years 2021–2050, the highest value of the examined standard deviation occurred in June (13.2 mm), while in the reference period, it occurred in July (11.8 mm) (

Table 2. Statistical characteristics of the water needs of the sweet cherry trees during particular months in 1981–2010 and 2021–2050.

Specification	Months
	April	May	June	July	August	September	From April to September
	1981–2010
Minimum (mm)	25	50	94	110	99	61	489
Maximum (mm)	53	86	124	158	136	95	587
Mean (mm)	37	70	108	131	119	77	542
Median (mm)	37	72	109	131	120	77	542
Standard Deviation (mm)	5.2	7.6	7.5	11.8	8.3	8.0	24.3
Variability Coefficient (%)	13.9	10.8	7.0	9.0	6.9	10.4	4.5
	2021–2050
Minimum (mm)	15	42	85	112	108	78	491
Maximum (mm)	58	94	135	159	157	107	656
Mean (mm)	33	64	109	139	134	93	573
Median (mm)	33	63	109	139	134	92	575
Standard Deviation (mm)	9.0	12.7	13.2	11.4	11.0	7.9	40.5
Variability Coefficient (%)	27.4	19.7	12.1	8.2	8.2	8.5	7.1

). Expressed by the variability coefficient, the relative diversity of sweet cherries’ water needs during the growing season was relatively small. The value of this parameter for the reference and forecast period was 4.5 and 7.1%, respectively. The highest monthly variability coefficient values for both of these periods were found in April (13.7% in the reference period and 27.4% in the forecast period).

Based on the calculations, it was found that in 2021–2050, the average daily water needs of sweet cherry trees in the months from June to September will be higher than in 1981–2010 (Figure 2a). In light of the predicted gradual increase in temperature, during the forecast period, the highest daily water requirement for sweet cherries will occur in July (4.5 mm). Generally, the sum of sweet cherry water needs in the entire growing season estimated in the forecast period (573 mm) is higher than in the reference period (542 mm) (Figure 2b).

Based on the calculations, it was found that during the entire vegetation period of sweet cherries, which begins on 1 April and ends on September 30, there was an upward trend in ETp, i.e., the water needs of sweet cherries (

Table 3. Statistical significance of the equation trends for the water needs of sweet cherry trees and their tendencies in 1981–2010 and 2021–2050.

Months	Period
	1981–2010	2021–2050
Linear correlation coefficient (r)
April	0.396 **	ns
May	ns	ns
June	ns	ns
July	ns	ns
August	ns	0.392 **
September	ns	ns
April–September	0.334 *	ns
Water needs tendency (mm·decade−1)
April	2.4	2.3
May	−1.0	2.2
June	2.0	3.2
July	3.2	0.6
August	1.7	5.0
September	1.1	−0.9
April–September	9.4	12.4

Explanations: * = significance for p = 0.1 (P = 90%); ** = significance for p = 0.05 (P = 95%); ns = not significant.

). In addition, it was also noted that the trend of time variability in the water needs of sweet cherries was significant only in the reference period. The water needs of sweet cherries in the growing season increase by 9.4 and 12.4 mm in each subsequent decade of the reference and forecast periods, respectively. A significant rising trend in the sweet cherries’ water needs was found only for two months of the growing season: in April in the reference period and in August in the forecast period.

The time trend of sweet cherries’ water needs and linear regression equations describing the tendency of the time variability of sweet cherries’ water needs, along with determination coefficients for the growing season (from 1 April to 30 September) as well as for April and August, are shown in Figure 3a,b, Figure 4a,b, and Figure 5a,b, respectively.

Based on the calculations, it was found that as a result of the climate change expected in the Bydgoszcz region in the years 2021–2050, there will be an increase in the water needs of sweet cherry trees, presented by the increase in potential evapotranspiration (

Table 4. Comparison of the water needs of sweet cherry trees in 1981–2010 and 2021–2050.

Period	Unit	Months
		April	May	June	July	August	September	From April to September
1981–2010	mm	37	70	108	131	119	77	542
2021–2050	mm	33	64	109	139	134	93	573
Difference (2021–2050)–(1981–2010)	mm	−4	−6	+1	+8	+15	+16	+31
	%	−11	−9	+1	+6	+13	+21	+6

). In the predicted period, during the growing season (April–September), an increase of 31 mm (6%) in the water needs of sweet cherries is expected. The largest monthly increase in water needs is expected in August (15 mm, 13%) and September (16 mm, 21%).

4. Discussion

In the climatic conditions of Poland, to provide the cherries with the right amount of water, the annual rainfall should be between 500 and 600 mm [8,30]. The average long-term totals of precipitation in the Bydgoszcz region are 520 mm [1,4]. According to Rzekanowski [4], the greatest rainfall deficits occur in the cultivation of fruit plants in the north-central part of Poland, in the so-called Land of Great Valleys. In the case of sweet cherries and cherries grown on medium-light soils, these deficits range from 20 to 42 mm. As stated by Kielak [31], the need to irrigate cherries in Poland may occur from mid-May to mid-August and may be as much as 140 mm, whereas Ostromęcki [12] reported that sweet cherry trees should be irrigated in the period from May to July.

In the present study, very high water needs of sweet cherries during the growing season (April–September) were found, amounting to 542 mm in the years 1981–2010 and 573 mm in 2021–2050. The explanation of this fact should be sought in the applied method of determining the water needs of sweet cherries according to Treder. Based on the calculations made using the Press method, it was found that the water needs of sweet cherries during the growing season (April–September) on medium-light soils range from 325 mm (during low temperature) to 400 mm (during high temperature) [11]. On light sandy soils, according to Press, the water needs of sweet cherry trees range from 390 to 480 mm, and on heavy, compact soils (loams and clays), the water needs of cherry trees are the smallest and range from 260 to 320 mm [11,12]. Treder [32] estimated that the water needs of sweet cherry trees in the Toruń region (about 50 km from Bydgoszcz) in the period from April to September amount to 459 mm. According to Treder [32], the greatest monthly water needs of sweet cherries occur in this region in June (103 mm) and July (108 mm). These values of sweet cherry water needs are comparable with the data estimated in the current study only for June, as the present research found much higher values of sweet cherry water needs of more than 130 mm in July. In addition, similarly high values of sweet cherry water needs also occurred in August in the present study.

According to the applied research methodology [27] and predicted climate changes [17,18], rainfall deficits during the sweet cherry growing season in the Bydgoszcz region in the reference period amounted to 232, 348, and 433 mm for normal (N_50%), medium dry (N_25%), and very dry (N_10%) years, respectively (

Table 5. Precipitation deficit (mm) for sweet cherry trees in normal, medium dry, and very dry years for the years 1981–2010 and 2021–2050.

Period	Months
	April	May	June	July	August	September	From April to September
Normal years
1981–2010	10	21	57	52	60	33	232
2021–2050	–	–	47	96	113	60	316
Medium dry years
1981–2010	19	38	78	81	83	49	348
2021–2050	–	19	70	118	131	76	414
Very dry years
1981–2010	25	50	93	102	101	61	433
2021–2050	5	32	88	137	146	89	495

). In the forecast period, the precipitation deficit increased to 316, 414, and 495 mm for normal, medium dry, and very dry years, correspondingly. It is worth noting that the precipitation deficiencies (N_50%), (N_25%), and (N_10%) ensure the water needs of sweet cherry trees at the level of 50, 75, and 90%, respectively, and these can be the basis for calculating the amount of water needed for irrigation and water tank volume for irrigation systems [11].

The increased water needs of sweet cherries found in the present study confirm the predicted rise in air temperature in the vegetation period. The increase in water needs is particularly noticeable in August. According to Łabędzki [15,16], an increase in air temperature by 2 °C or even 4 °C can be expected in Poland. In turn, the rise in air temperature results in an increase in the water needs of plants [15,16,17,18]. Temperature measurements in the Bydgoszcz region conducted for over 80 years have shown a clear upward trend in the average annual temperature; it was found that the air temperature increased by 0.19 °C per 10 years, i.e., by almost 2.0 °C per 100 years [33]. It is considered very likely that, with the exception of the Baltic coast, the average monthly air temperature, both in July and August, may even be higher than 25 °C [15]. However, it should be mentioned here that the climate change scenarios developed for Poland until 2050 and 2080, which were prepared on the basis of different models, differ significantly, especially when we take into account the summer period, i.e., June–August. During this period, there is a significant increase in the water needs of plants. When analyzing various climate change scenarios, it can be seen that all of them unanimously predict a rise in air temperature, but only a few of them mention an increase in precipitation. There are also models that suggest a reduction in the amount of precipitation in Poland [15]. On the one hand, an overwhelming number of climate change scenarios developed for Poland suggest that in the future there will be no rise in precipitation during the year. On the other hand, studies indicate that there may be an increase in the amount of precipitation during the winter months and at the same time a reduction in the amount of rainfall during the summer months [13,14,34,35,36,37,38].

Undoubtedly, both the global warming we are currently observing and the global warming projected in the future will increase the water demands of plants [17,18,39,40,41], which is confirmed by the results of the evaluation of the water needs of sweet cherries presented in this paper. Therefore, there is a justified need (and even necessity) to undertake a number of adaptation actions. Supplemental irrigation is one of them. A number of studies have already confirmed the fact that along with the deepening of global warming, the importance of supplementary irrigation in Poland is increasing, especially in north-central Poland, including the Bydgoszcz region [15,16,21,42,43,44,45,46,47,48,49,50,51]. It is believed, however, that both the amount of water used for the irrigation of agricultural crops and the area of irrigated crops in Poland will depend mainly on the economic conditions of agriculture and its development strategy and to a much lesser extent on climatic conditions [16].

Positive production effects in the cultivation of cherries and sweet cherries in Poland are the result of the use of sprinkling irrigation [31], drip irrigation [1,5,52], and under-crown irrigation [53]. Pacholak [54] reported that in Poland, thanks to the use of irrigation, the yield of cherries may increase from 19.3 to 25.7%, while it was found that the fertility of trees also depends on the dose of irrigation and the amount of rainfall, as well as on the cultivar and growing region. Lipiński [52] showed that in the research conducted in 2013, cherry yields were on average 10.2 t ha⁻¹ (from 3.0 to 14.0 t ha⁻¹) in non-irrigated orchards and 12.27 t ha⁻¹ (from 3.5 to 15.0 t ha⁻¹) in irrigated orchards. The increase in yields due to irrigation was on average 2.07 t ha⁻¹ (20%), fluctuating within the range of 1.6 to 2.6 t ha⁻¹ (i.e., 11–29%). In an experiment conducted in north-western Poland [55,56], an increase in yield of 10.3% due to irrigation was recorded in a normal year or a year with slightly higher precipitation, and in a dry year, the increase in yield was 59.6%. In a study by Kielak [31], the yield increase due to irrigation in a dry year amounted to as much as 123%, which was achieved in a young orchard in the fifth year after planting the trees.

In our research, the water needs of sweet cherries, determined as potential evapotranspiration, were estimated using the plant coefficient method. In this method, calculations were made on the basis of reference evapotranspiration. It is believed that this is the optimal method for estimating the water needs of plants [23,57,58]. Reference evapotranspiration was determined according to the Treder method [24,25]. The choice of this method was supported by the fact that it is based on changes in air temperature. The rise in temperature, entailing an increase in water needs, was analyzed by the authors of the publication which was used in our study to perform reference evapotranspiration calculations [18]. In order to calculate the water needs of sweet cherries, other methods have also been used, which have been described in more detail by Rolbiecki [25], but we did not compare them in this study because they are not of a methodical nature, i.e., they do not compare many methods useful for estimating the water needs of sweet cherries. However, in future research, it would be useful to carry out a comparative analysis of several methods of estimating the water needs of sweet cherries. Such studies should be carried out for more climate change scenarios, and the studies should include not only other regions of Poland but also other countries of Central and Eastern Europe. Such research would allow us to obtain more objective results, additionally characterized by a wider range of their applications.

There is little scientific literature on the water needs of sweet cherry trees, but some of them include the results of sap flow measurements in cherry orchards in Hungary and South Africa, New Zealand, and Oceania [59,60,61]. These investigations were carried out locally on individual trees to estimate water use in specific growth stages and weather conditions and are useful for planning irrigation.

5. Conclusions

The projected global increase in air temperature, which is associated with a rise in the water needs of crops, will increase the demand for supplementary irrigation treatments. This study estimated the water needs of sweet cherry trees in the years 2021–2050 in the region of Bydgoszcz, Poland. The calculations carried out in this study showed an increase in the water needs of sweet cherry trees in the vegetation period, which begins on 1 April and ends on 30 September, from 542 to 573 mm (an increase of 31 mm, i.e., 6%) should be expected. The largest and most significant increase in the water needs of sweet cherries, among the analyzed months of the growing season, should be expected in August (15 mm, 13%) and September (16 mm, 21%). In addition, the trend of the time variability of sweet cherries’ water needs in August shows a rise of 5 mm in each subsequent decade of the forecast period (2021–2050). The results obtained in this research will be useful in the design and planning of supplementary irrigation treatments for sweet cherry orchards located in the Bydgoszcz region. The results can also be useful in practice to evaluate the potential crop production (cherries) and potential water use on the basis of the specific climate scenario.