Over the period 1981–2020, the Southeastern River basin experienced a minimum annual ETa of 842.04 mm in 1984 and a maximum value of 949.34 mm in 2012. Meanwhile, the ETa exhibited a significant increasing trend (Fig. 3A) with an accelerated rate of 2.51 mm/yr (p < 0.05). The results depicted in Fig. 3A demonstrate a relatively substantial increase in ETa between 1981–2000 and 2001–2020, and it also indicates that there was an abrupt change in ETa around 2000, with mean ETa rising from 868.29 mm in 1981–2000 to 927.16 mm in 2001–2020. The average ETa showed an increase from 868.29 mm in 1981–2000 to 927.16 mm in the years spanning from 2001–2020, with a transition from negative to positive ETa anomalies occurring in the year 2000 (refer to Fig. 3B).
3.1.2 Spatial Variations
The spatial variability of multi-year average ETa in the Southeastern River basins exhibited significant differences, ranging from 534.63 to 1345.96 mm. Meanwhile, the ETa in the Southeastern coastal region exhibits a dominant increasing trend with an average annual increase of 2.51 mm/yr (Fig. 5). The rate of increase is higher in the Southeastern region, and a decreasing trend is observed in the northern part. The histogram indicates that most grids experience an ETa increase rate ranging from 1.5 to 3.5 mm/yr, which corresponds well with the spatial pattern of ETa.
The spatial variation of ETa in the basin of the Southeastern River showed significant seasonal differences. In winter, the basin-wide ETa tended to increase, except for individual grid points; however, the rate of increase was relatively low at less than 1 mm/yr (Fig. 6A). Notably, the southern region exhibited a faster rate of increase than its northern counterpart due to higher temperatures during winter. In most areas, the ETa rate increased by 1–2 mm/yr during spring (Fig. 6B). The central part of the basin experienced a higher rate of increase than other regions. In summer, the basin experienced an increase in ETa, with a rate of over 1 mm/yr at most grids (Fig. 6C), similar to spring (Fig. 6B). During autumn, the central and southern regions of the basin saw a faster increase in ETa than other areas. However, some parts of the northern region witnessed a lower rate of increase or even decrease (Fig. 6D).
3.1.3 ETa Components changes
The GLEMA ETa component comprises five primary constituents: transpiration (Et), forest canopy interception (Ei), water evaporation (Ew), bare ground evaporation (Eb), and snow sublimation (Es). The amount of Es is negligible due to the low latitude of the Southeastern River basin and minimal seasonal snow accumulation only occurring in the local northern area 37. Hence, Et, Ei, Ew, and Eb are analyzed in this study. The predominant contributor to ETa in the Southeastern River basin is Et, accounting for 73.45%, followed by Ei at 18.26%. In the past four decades, Et, Ei, Ew, and Eb in the Southeastern River basin have exhibited an increasing trend (Table 1), with Ei, Ew, and Eb all passing the significance test at a 99% confidence level. By comparing the changes in ETa between 1981–2000 and 2001–2020, it was observed that there was a significant increase of 58.87 mm in ETa during the latter period compared to the former. This increase can be attributed to Ei by 18.49 mm (31.41%) and Et by 27.25 mm (46.28%). Both Ew and Eb exhibit an increasing trend, albeit relatively small. Therefore, the primary contributors to the increase of ETa in the Southeast River basin over the past 40 years are Et and Ei, with vegetation being one of several influential factors that warrant attention.
Table 1
Multi-year mean values and trends of each component of ETa in the basin.
Component
|
1981–2020(mm)
|
T1: 1981–2000(mm)
|
T2: 2001–2020(mm)
|
T2-T1
|
Trend(mm/yr)
|
ETa
|
897.73
|
868.29
|
927.16
|
58.87
|
2.51**
|
Et
|
659.36
|
645.77
|
673.02
|
27.25
|
0.87
|
Ei
|
163.97
|
154.73
|
173.22
|
18.49
|
0.93**
|
Ew
|
51.33
|
50.07
|
52.59
|
2.52
|
0.09**
|
Eb
|
19.27
|
17.72
|
20.83
|
3.11
|
0.13**
|
Note: * indicates passing the significance test with a 90% confidence level, ** indicates passing the significance test with a 95% confidence level, and *** indicates passing the significance test with a 99% confidence level. |
The Et showed significant growth in most of the basin, and a negative growth trend only appeared in the northern and eastern coastal regions (Fig. 7A). The regional variation of Ei is similar to but smaller than Et (Fig. 7B). Figure 7C shows that Ew changes are relatively dispersed. Eb is opposite to the distribution of Et and Ei, forming a nearly complementary spatial pattern (Fig. 7D). In general, the change distribution of Et and Ei is consistent with that of ETa, which further indicates that Et and Ei are the main contributing factors to the growth of ETa in the southeast river basin.
Regarding proportional changes in components, Fig. 8 demonstrates that the interannual variations in Ei, Ew, and Eb proportions are consistent with ETa. The trends of Ei, Ew, and Eb ratios generally align with those of ETa. However, the increments of Et and its proportion exhibited opposite trends, particularly after 2000. The proportion of Et experienced a more pronounced decrease, while Ei demonstrated a significant upward trend during the same period. The Ei increase partially offset Et's declining trend; thus, ETa continued to exhibit an increasing trend.
3.2 Influencing factors in ETa change
To further investigate the impact of ETa changes during 1981–2020 in the South-eastern River basin, we conducted a correlation analysis between ETa and climate factors, including temperature (T), wind speed (Win), relative humidity (Rhu), and vegetation index (NDVI). The results of correlation analysis (Table 2) indicate that temperature, relative humidity, and vegetation are significant factors affecting ETa variation in the Southeastern River basin. It is supported by strong correlations between ETa and T (r = 0.76, p < 0.01), Rhu (r=-0.72, p < 0.01), and NDVI (r = 0.78, p < 0.01).
Table 2
Pearson’s correlation coefficient shows the ETa and meteorological factors in the South-eastern River basin.
Factors
|
T
|
Win
|
Rhu
|
NDVI
|
1981–2020
|
0.76***
|
0.13
|
-0.72***
|
0.78***
|
Note: * indicates passing the significance test with a 90% confidence level, ** indicates passing the significance test with a 95% confidence level, and *** indicates passing the significance test with a 99% confidence level. |
Meanwhile, the contribution of climate factors to changes in ETa was calculated using a multiple regression model. R2 of the regression equation was 0.89, which indicates that the multiple regression model simulation was effective. 2001–2020 witnessed a significant increase in T, Win, and NDVI compared to 1981–2000, while Rhu experienced a notable decrease. This shift change resulted in a marked rise in ETa from 2001 to 2020. Table 3 indicated that temperature (T) is the primary contributor to ETa, accounting for 47.93% of the total change, followed by relative humidity at 40.45% and NDVI at 10.66%. These three factors account for a total contribution of 99.11% to ETa in the Southeast River basin since 2000. Although the contribution of NDVI is only 10.66%, its spatial changes (Fig. 9) exhibit a similar pattern to that of ETa (Fig. 5).
Table 3
Contribution of each influencing factor to ETa increase (%).
Factors
|
ETa
|
T
|
Win
|
Rhu
|
NDVI
|
T1: 1981–2000
|
0.25
|
0.33
|
0.41
|
0.81
|
0.34
|
T2: 2001–2020
|
0.79
|
0.69
|
0.49
|
0.37
|
0.62
|
Amount of Change
|
0.54
|
0.36
|
0.08
|
-0.44
|
0.28
|
Regression Coefficients
|
|
0.63
|
0.06
|
-0.44
|
0.18
|
Actual Contribution Rate /%
|
|
47.93
|
0.96
|
40.45
|
10.66
|
3.3 The relationship between Changes in ETa and Seasonal Drought
ETa plays a crucial role in triggering drought events. The results depicted in Fig. 10 demonstrate a strong correspondence between the variation of ETa anomaly and alterations in both soil moisture (SM) and SPEI index. From 1981 to 2000, negative ETa anomalies corresponded to soil moisture-positive anomalies, while SPEI changes remained relatively small, indicating the absence of drought. However, from 2001 to 2020, positive anomalies in ETa were accompanied by negative anomalies in soil moisture, leading to a significant increase in the volatility of SPEI and indicating an escalating frequency of drought events. ETa changes are closely related to drought in the Southeastern River basin.
The spatial correlation among ETa, SM, and SPEI was analyzed, and the results are shown in Fig. 11. From 1981 to 2020, the Moran Index of ETa and SM was − 0.194, showing a negative correlation (Fig. 11A). Regarding clustering distribution (Figs. 11B and C), significant negative correlations occurred in the north (Low-High correlation) and south (High-Low correlation). At the same time, ETa and SPEI also showed a highly significant negative spatial correlation with a Moran index of -0.513 (Fig. 11D). The cluster distribution (Figs. 11E and F) indicated that the Low-High correlation occurred in the north part, and the High-Low correlation was in the central and south regions. Therefore, the increase in ETa is an essential driving factor for drought in this region.
Meanwhile, we conducted a detailed analysis of two representative years (2003 and 2011) to investigate their correlation further. The changes observed in ETa during January-March and May-September of 2003 were contrary to those seen in SM and SPEI, as depicted in Fig. 11. In July, the positive ETa anomaly was the largest, corresponding to SM, and SPEI showed a negative anomaly. It suggests that high ETa rates and low precipitation in July 2003 resulted in soil moisture depletion and triggered seasonal drought. Despite a negative ETa anomaly in December, SM and SPEI exhibited negative anomalies. It indicates that although winter ETa values were relatively low, they were susceptible to drought due to insufficient precipitation.
The variation in ETa observed in 2011 was consistent with the corresponding changes in SM, as illustrated by Fig. 13. ETa exhibits a positive anomaly in April, while the corresponding SM and SPEI exhibit negative anomalies. From June to August, ETa exhibits a positive anomaly. However, the corresponding changes in SM and SPEI are relatively small, indicating that precipitation during this period may mitigate the negative impact caused by the increase of ETa. ETa showed a negative anomaly in November, whereas SM displayed a minor variation. The positive anomaly of SPEI indicated that the low ETa significantly contributed to its deviation from normalcy during that month.
Therefore, the SPEI was lower in 2011 than in 2003 for the whole year but higher in spring than in 2003, indicating that the possibility and severity of seasonal droughts like spring drought in the region are increasing with ETa increased.