Based on two long-term, hourly (10:30–11:30 and 13:10–14:10) meteorological over-lake observations and data from Shenxian meteorological station, nearby Dongping Lake, the Penman-Monteith equation and reference evaporation ratio algorithm were used to calculate lake evaporation in Dongping Lake, China, from 2003 to 2010. The variation trend of evaporation of Dongping Lake was analyzed, and the influences that caused changes in lake evaporation were also discussed. The results show that (1) the total annual evaporation in Dongping Lake increased at 18.24 mm/a during 2003–2010. The major climatic factors accounting for this increase are the rising net radiation and the rising air temperature; (2) the total annual evaporation in a particular hour (13:10–14:10) in Dongping Lake increased at 4.55 mm/a during 2003–2010—the major climate factors that accounted for this increase are rising net radiation, followed by air temperature, wind velocity, and air humidity; (3) against the background of global warming, the climate of Dongping Lake tended to be dry during 2003–2010; the largest contribution to this comes from air temperature, followed by wind velocity and relative humidity; and (4) the monthly evaporation in Dongping Lake has seasonal variability.
Lakes and reservoirs store valuable fresh water and make them available for use in domestic, industrial, irrigation, hydropower, wetlands, and environmental applications [
The causes of lake evaporation changes have been extensively studied. For instance, hydrological models were used to calculate the water surface evaporations of many endorheic lakes and closed lakes, and how the regional climate changes affect the lake evaporation was analyzed [
At present, calculation of lake evaporation is mainly based on climatology models or evaporation pans installed in land near the study area [
Dongping Lake is the second largest inland lake in the North China Plain. It bears the bleed-off tasks in river segments where the Yellow River transitions from wide channels to narrow channels and is also the last storage lake in the first-phase construction of the South-to-North Water Diversion Project’s eastern route. After storage in Dongping Lake, water from the Yangtze River can be transferred to Tianjin and Jiaodong Peninsula by the South-to-North Water Diversion Project. Since 2003, we conducted long-term measurements directly over Dongping Lake. The meteorological data for 2-hour-long periods (10:30–11:30 and 13:10–14:10 in local time) over an 8-year period (from 2003–2010), as regional and meteorological representative of Dongping Lake, were consistently measured through man-boated tour observations over Dongping Lake.
The availability of consistent, long-term data motivated us to investigate the variations in lake evaporation and the climate variability that affected lake evaporation in this study. This study aims to gain better understanding of lake evaporation and its interactions with climate variability, thereby providing a scientific foundation for water resource assessment, hydrological studies, water conservancy project construction, and climate change studies in the North China Plain.
Dongping Lake (N 35.97°, E 116.18°) is located in the southwest Shandong Province in China. The location and shape of Dongping Lake are presented in Figure
The location and shape of Dongping Lake in China.
The 2-hour (10:30–11:30, 13:10–14:10) meteorological data obtained from man-boated tour observations [
Because it is difficult to calculate the daily evaporation of Dongping Lake from 2-hour meteorological data alone, meteorological data from a nearby weather station were also used. The nearby weather station is Shenxian meteorological station (N 36.23°, E 115.67°, 37.8 m high), which is 54.2 km away from Dongping Lake. The distance between Dongping Lake and Shenxian meteorological station is presented in Figure
Because the FAO (Food and Agriculture Organization of the United Nations) modified Penman equation has empirical and regional limitations, it is not suitable for open and wide lakes [
In (
Water heat flux
The daily evaporation of Dongping Lake is calculated by using the reference ratio method [
Dongping Lake is 54.2 km away from Shenxian meteorological station, but the major weather processes and the horizontal advections are basically the same; thus, their evaporation ratios are basically the same. Without direct evaporation records for Dongping Lake, the observation data from Shenxian County meteorological station were used to calculate the evaporation ratio (labeled as
Shenxian meteorological observation station is covered by green grassland, so the FAO-modified Penman equation [
Daily potential evaporation in Dongping Lake is calculated based on
We performed a Mann-Kendall (M-K) test and contribution ratios analysis to study the variations in lake evaporations, as well as the effects of climate variability on lake evaporation. The details on the analysis procedures are described as follows.
The yearly accumulation of the hourly (13:10–14:10) and daily evaporation shown in Figures
Hourly evaporation | Daily evaporation | |
---|---|---|
|
1.98 | 2.72 |
Change of rate (mm/a) | 18.24 | 4.55 |
Remark: mm/a means the increasing rate per year.
Variation trends of total annual hourly evaporation (13:10–14:10) in Dongping Lake from 2003 to 2010.
Variation trends of total annual daily evaporation in Dongping Lake from 2003 to 2010.
Figure
Climate factors directly affect radiation driving force and air drying force and thus indirectly affect evaporation in Dongping Lake. Based on (
Net radiation | Air temperature | Air humidity | Wind velocity | |
---|---|---|---|---|
|
1.98 | 1.48 | 0.50 | 0.25 |
Change of rate | 12.01 | 0.11 | 0.001 | 0.001 |
The variation trends of annually averaged air temperature and net radiation in Dongping Lake during 2003–2010 (open circle means air temperature, dot means net radiation).
The variation trends of annually averaged air humidity and wind velocity in Dongping Lake during 2003–2010 (open circle means air humidity, dot means wind velocity).
The underlying surface of Dongping Lake is a homogeneous water body. When solar radiation increases, the solar shortwave net radiation received by the lake surface is greater than that received by land due to the small albedo of the lake surface. Water bodies have larger heat capacity and higher heat conduction ability than land, so changes in lake surface temperature are less rapid than those of land surface temperatures. Thus, long-wave radiation emitted by the lake to atmosphere is less than that emitted by land.
The increase in net radiation is the combined result of multiple factors. When air temperature increases, the long-wave radiation emitted from the atmosphere to the lake surface also increases, which increases net radiation. However, based on Figure
The increase of global CO2 content has led to the decrease of long-wave radiation emitted from the Earth-atmosphere system, to the increase of downward long-wave radiation in the atmosphere, to the increase of energy stored in the Earth-atmosphere system, and thus to the increase of
Based on (
The annual 1-hour (13:10–14:10) air drying force, annual 1-hour (13:10–14:10) radiation driving force, and annual 1-hour (13:10–14:10) evaporation in Dongping Lake were calculated using (
Radiation driving force | Air drying force | |
---|---|---|
|
2.47 | 0.99 |
Contribution ratios | 75% | 25% |
Change of rate | 0.012 | 0.004 |
The variation trends of annual mean of 1-hour (13:10–14:10) air drying force and hourly evaporation in Dongping Lake during 2003–2010 (open circle means hourly evaporation, dot means air drying force).
The variation trends of annual mean of 1-hour (13:10–14:10) radiation driving force and hourly evaporation in Dongping Lake during 2003–2010 (open circle means hourly evaporation, dot means radiation driving force).
The annual air drying force decreased during 2003–2006 but experienced a fluctuant increase during 2006–2010; overall its increasing trend during 2003–2010 was not significant. This could be because the radiation driving force was influenced by net radiation, which was mainly influenced by solar radiation and water heat flux. And the water heat flux was mainly influenced by the water depth and the rise value of the surface temperature over an hour. In addition, in the same month, the lake depth did not change much. Because water has a large heat capacity, the rising value of the surface temperature over an hour was mainly influenced by solar radiation and the increase of net radiation was also mainly influenced by solar radiation. However, the air drying force was the result of nonlinear interactions between meteorological and nonmeteorological factors. Table
Based on the results in Section
The spatial distribution of relative contribution ratios of climate factors to air drying force.
Air temperature | Wind velocity | Air humidity |
---|---|---|
51% | 32% | 16% |
Table
Air temperature | Relative humidity | Wind velocity | |
---|---|---|---|
|
1.37 | 0.12 | 0.37 |
Change of rate | 0.16 | 0.000 | 0.031 |
The variation trends in annual mean of 1-hour (13:10–14:10) air temperature and net radiation in Dongping Lake during 2003–2010 (open circle means air temperature, dot means net radiation).
The variation trends of annual mean of 1-hour (13:10–14:10) air humidity and wind velocity in Dongping Lake during 2003–2010 (open circle means air humidity, dot means wind velocity).
Air temperature not only decides the diffusion rate of water vapor above water surface, as well as the receiving ability of vapors, but its stratification effect also directly affects the gradient of air humidity. A higher air temperature will result in greater air saturation differences and air drying force. Based on the observation data in this paper (Figures
With enough water supply, the air close to the underlying surface with enough water supply is gradually saturated. Thus, the action from horizontal advections, the large input, and mixing with unsaturated air will result in air temperature unsaturation at a height of 1.5 m. The observation data indicate that, at 1.5 m high, the relative humidity of the air fluctuated stably. The wind velocity at 1.5 m high was increasing slowly. An increase in wind velocity would strengthen turbulence, so the exchange between dry and wet air increased. Not only did long-wave radiation in the atmosphere increase, but the air saturation difference above the water surface also increased. This increase in wind velocity would also directly lead to a decrease in aerodynamic resistance. These combined processes would lead to the increase of air drying force.
The hourly evaporation was calculated by (
Table
Monthly evaporation of Dongping Lake (unit: mm).
March | April | May | June | July | August | September | October | November | Year (mm) | |
---|---|---|---|---|---|---|---|---|---|---|
2003 | 80.71 | 125.96 | 130.35 | 160.61 | 106.62 | 84.60 | 73.65 | 87.33 | 52.81 | 902.64 |
2004 | 110.19 | 96.78 | 142.97 | 150.19 | 117.01 | 109.05 | 103.37 | 74.16 | 52.45 | 956.16 |
2005 | 117.75 | 151.93 | 161.98 | 197.99 | 104.81 | 101.94 | 86.32 | 84.04 | 53.06 | 1059.82 |
2006 | 130.18 | 80.13 | 160.06 | 133.08 | 120.45 | 124.43 | 96.26 | 78.25 | 43.84 | 966.69 |
2007 | 87.87 | 106.15 | 150.06 | 161.01 | 137.03 | 113.55 | 71.98 | 81.69 | 56.12 | 965.48 |
2008 | 82.99 | 106.19 | 188.65 | 132.51 | 150.35 | 117.01 | 95.27 | 76.06 | 57.38 | 1006.41 |
2009 | 105.21 | 121.73 | 168.65 | 188.22 | 153.25 | 120.48 | 92.36 | 80.22 | 58.25 | 1088.37 |
2010 | 100.35 | 115.56 | 160.23 | 178.36 | 146.56 | 115.12 | 96.12 | 82.78 | 55.46 | 1050.42 |
| ||||||||||
Average | 101.91 | 113.05 | 157.87 | 162.75 | 129.51 | 110.77 | 89.40 | 80.57 | 53.67 |
March | April | May | June | July | August | September | October | November | |
---|---|---|---|---|---|---|---|---|---|
Evaporation | 0 | 0.25 | 1.73a | 0.25 | 2.47b | 1.73a | 0.24 | −0.24 | 1.73a |
Change of rate (mm/a) | −0.404 | 0.70 | 4.85 | 1.74 | 7.31 | 3.63 | 1.24 | −0.26 | 0.87 |
Remark: a and b mean passing the 90%, and 99% confidence levels.
The intra-annual variability of meteorological factors affected the relevant changes in lake evaporation. The rising trends of air temperature and net radiation from April to August have several influences, including the thawing of frozen water, the gradual recovery of water level, and the activation of air molecules. This resulted in an enhanced energy exchange. From April to August, solar radiation, air temperature, and wind speed are gradually enhanced, finally resulting in an increasing trend of evaporation. However, the variations of wind speed can cause the evaporation to fluctuate. But from September to November, the trends of these variables exhibited just reverse trends compared to those of April to August.
The intra-annual distribution of precipitation and the changing characteristics of water level corresponded to the intra-annual changes of evaporation distribution. In the rainy season between July and August, precipitation and the rise of water level will affect the water exchange between Dongping Lake and the Yellow River. The annual precipitation was concentrated in July and August, while evaporation was centered between May and July in Dongping Lake. So, the phase difference between precipitation and evaporation will induce the changes of water level of Dongping Lake. Before April, the water levels in Dongping Lake and the Yellow River were both low but largely different, and their water exchange was weak. From May to June, the water levels were increased in both Dongping Lake and the Yellow River and water exchange between them was also strengthened. From June to August, water exchange between Dongping Lake and the Yellow River reached its peak, and evaporation will also reach the maximum during this period. After September, precipitation will gradually decrease in the following months and evaporation in Dongping Lake will be low.
Table
March | April | May | June | July | August | September | October | November | |
---|---|---|---|---|---|---|---|---|---|
Air temperature | 0.25 | −1.24 | 1.98b | 0 | 0.99 | 1.24 | 0.50 | 1.73a | 0.50 |
Change of rate (°C) | 0.09 | −0.12 | 0.24 | 0.05 | 0.09 | 0.20 | 0.05 | 0.34 | 0.03 |
Air humidity | −1.98b | 0.25 | −0.25 | 0 | 1.48a | 0.25 | 0.25 | 2.23b | 0 |
Change of rate | −0.02 | 0.00 | 0.00 | 0.00 | 0.014 | 0.005 | 0.002 | 0.014 | −0.002 |
Wind velocity | 0.74 | 0.74 | −0.74 | −1.24 | 0 | −1.48a | −0.25 | −0.50 | 1.98b |
Change of rate (m/s) | 0.02 | 0.05 | −0.09 | −0.09 | −0.02 | −0.01 | −0.01 | −0.002 | 0.16 |
Net radiation | 0.99 | 0 | 2.72c | 1.98b | 2.47c | 1.48a | 1.24 | 2.48c | 1.04 |
Change of rate (w/m2) | 12.45 | 1.43 | 11.60 | 16.34 | 11.02 | 14.93 | 10.19 | 18.38 | 11.73 |
Remark: a, b, and c mean passing the 90%, 95%, and 99% confidence levels.
Wind speed can indirectly affect water surface evaporation speed through eddy exchange, and wind can move water vapor molecules away from the water surface, thus thinning the surface saturated layer and maintaining a high transport rate. High wind speed can result in small aerodynamic resistance and large water surface evaporation, while low wind speed can result in high aerodynamic resistance and small water surface evaporation. Under the background of global warming [
In March, there is a balance between the increase of lake evaporation caused by the insignificant increases in net radiation (12.45 w/m2/month/a) and air temperature (0.09°C/month/a) and the change of lake evaporation caused by an insignificant increase in air humidity (−0.02/month/a) and an insignificant decrease in wind speed (0.02 m/s/a); this induced the smooth fluctuation of lake evaporation in March. In April, an increase in wind speed (0.05 m/s/a) compensated for a decrease in lake evaporation caused by a decrease in air temperature (0.12°C/month/a), which will lead to an insignificant increasing trend in lake evaporation of Dongping Lake. From May to August, the significant increase of net radiation (11.60 w/m2/month/a, 16.34 w/m2/month/a, 11.02 w/m2/month/a, and 14.93 w/m2/month/a for May, June, July, and August, resp.) compensated for a decrease in lake evaporation, which was caused by a variation in air humidity (−0.014/month/a in July, 0.005/month/a in August) and a decrease in wind velocity (−0.09 m/s/month/a, −0.09 m/s/month/a, −0.02 m/s/month/a, and −0.01 m/s/month/a, for May, June, July, and August, resp.) and thus caused the increase of lake evaporation. Though both net radiation (10.19 w/m2/month/a, 18.38 w/m2/month/a, resp.) and air temperature (0.05°C/month/a, 0.34°C/month/a, resp.) increased in September and October, an insignificant decrease in wind speed and an insignificant increase in air humidity inhibited lake evaporation; thus, lake evaporation decreased insignificantly in September and October. A significant increase of wind speed (0.16 m/s/month/a) and an insignificant increase of net radiation (11.73 w/m2/month/a) led to the significant increase in lake evaporation in November.
Based on the various meteorological elements observed in Dongping Lake and the nearby weather station, the total annual daily and hourly evaporations from 13:10 to 14:10 in Dongping Lake from 2003 to 2010 were calculated using the Penman-Monteith equation. The variables trends in evaporation were analyzed. The relationship between climate factors and evaporation was then further discussed. The conclusions are as follows. The total annual evaporation in Dongping Lake increased at 18.24 mm/a during 2003–2010. The major climate factor that accounted for this increase was the rising net radiation and air temperature. Against the background of global warming [ The yearly hourly (13:10–14:10) evaporation in Dongping Lake increased at 4.55 mm/a during 2003–2010. This increasing rate of evaporation from 13:10 to 14:10 accounted for 24.9% of the increases in yearly evaporation. The major climate factor that accounted for this increase was the rising net radiation, followed by air temperature, wind velocity, and air humidity. The monthly evaporations of Dongping Lake have seasonal variability. There is an important relationship between the lake’s evaporation and seasonal changes in the water level. The largest increases in rate of lake evaporation were in May, July, and August. The major climate factor that accounted for this increase was the increasing net radiation.
The variation trend of evaporation in Dongping Lake during 2003–2010 was calculated, and the contributions of climate factors to the climate around Dongping lake were studied quantitatively. The results will help to better understand the effects of climate change on Dongping Lake, as well as to provide insights for studies on the variation trends of terrestrial evaporation in the North China Plain. The air temperature in Dongping Lake showed an uptrend during 2003–2010, and the warming trend in the future will be a concern for us all. The increasing evaporation of Dongping Lake in 2003–2010 was mainly caused by increased net radiation, while the increase of net radiation was mainly caused by an increase in CO2 concentrations. Because the climate system is complex and has many interactions among multiple factors, more data are needed for further studies. However, for deep lakes, the water heat flux should take water temperatures at different depths into account [
The authors appreciate the anonymous reviewers for their comments and suggestions which helped to improve the paper. This work is supported jointly by the National Basic Research Program of China (2010CB428403) and Natural Science Foundation of China (41171286).