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Chapter 19. Potential evaporation and field water consumption in the north China plain
Institute of Geography, Academia Sinica, Beijing, China
THIS PAPER ESTIMATES potential evaporation and field water consumption in the North China Plain. In order to provide a basis for considering water transfer from the Chang Jiang, a rough estimate of the water deficit is calculated from the relationship between precipitation and evaporation.
METHOD FOR ESTIMATING POTENTIAL EVAPORATION
A great deal of work has been done on potential evaporation by hydrometeorological researchers in China (Zhu and Yang, 1955; Lu et al., 1965; Qian and Lin, 1965; Gao et al., 1978). This research commonly uses the formulae of Penman or Budyko. Here we use an empirical formula obtained by the Institute of Geography from experimental data to estimate potential evaporation in the North China Plain (Cheng and Cheng, 1980). This formula is
E0 = 0.19(20 + T)2 (1 - R) (1)
where E0 is potential evaporation (in mm), T is mean monthly temperature (°C) and R is monthly mean relative humidity (per cent).
The annual value of E0 estimated by formula (1) tends to be about 7 per cent lower than the observed values from a large evaporation pond, so it approximates the value of evaporation from a water surface. The Penman formula yields slightly higher results than equation (1) but is basically the same.
The advantage of the empirical formula lies in its relative convenience. With it the observed data of a vast number of meteorological stations can be used to study the distribution of potential evaporation at meso- and microscales.
ESTIMATES OF POTENTIAL EVAPORATION
Using the empirical formula, we estimated potential evaporation for 360 stations in and adjacent to the North China Plain and for 46 representative stations in the plain on a yearly and monthly basis from after 1949 to 1978. The average tong-term value in the North China Plain is about 700 to 950 mm (Figure 1) and is distributed as follows:
(1) There are marked geographical variations due to differences in the waterheat conditions of the underlying surface, the intensity of solar radiation and the radiation balance values which are influenced in turn by the surrounding mountains and seas as well as by lakes, rivers and extensive irrigation in the plains. The minima occur in the coastal plains, about 700 mm/annum south of the TianjinShanhaiguan railway line, 700 to 800 mm in northern Shandong Province and below 700 mm in northern Jiangsu Province.
Figure 1. Distribution of Annual Evaporation Capacity
Potential evaporation is higher in the piedmont plain, 800 to 900 mm. It reaches 900 to 950 mm in northwest Shandong and the vast tract around Nangong, Hengshui and Hejian in Hebei. This is a centre of intense evaporation and is the area of most serious drought and water deficit in north China.
(2) Seasonally, potential evaporation is low and stable from late autumn to early spring. Only about 18 per cent of the total occurs in the five months from October to February, with a minimum in January.
After spring begins, evaporation intensifies remarkably with the rise in temperature, scarce precipitation, dry air and strong winds typical of North China. Potential evaporation from March to June accounts for about 52 per cent of the year's total, with a peak value in June. It drops during the subsequent rainy season from July to September, when it constitutes only 30 per cent of the annual total.
(3) The difference in potential evaporation between the highest and lowest months is 80 to 100 mm/annum in the coastal plain, reaching 90 to 100 mm/annum in the coastal areas of northern Shandong and to the south of the TianjinShanhaiguan railway line. The largest difference is in the Heilonggang area, 150 to 160 mm/annum.
Year-to-year variation is also pronounced. For example, in Dezhou in Shandong, average potential evaporation from 1952 to 1978 was 947.2 mm/annum ranging from only 642.4 mm in 1964 to 1,222.4 mm in 1968, a difference of 90 per cent.
ESTIMATE OF FIELD WATER CONSUMPTION IN THE NORTH CHINA PLAIN
Field water consumption is the sum of transpiration and soil evaporation between plants. It is also called field evapotranspiration. It is one of the most difficult water balance elements to determine because of the threefold influences of biology, soils and weather.
Numerous methods are available for determining and estimating field evaporation. These methods fall into the following general categories: heat balance methods, aerodynamic (gradient) methods, vorticity correlation methods, water balance methods, measurement with evaporimeters and the use of empirical coefficients. The Institute of Geography carried out a comparative study of all these methods except vorticity correlation.
It was felt that the measuring error for the gradient method is the highest, with a difference of 20 to 100 per cent from evaporimeter measurements. This method needs to be improved and its suitability for China is pending further study.
The energy balance method has a good theoretical basis in the concept of energy conservation, but it is difficult to obtain accurate observations when the sky is overcast or cloudy or when there is a temperature inversion. In recent years, the accuracy of observation abroad has been greatly improved by the application of computer techniques and the installation of automatic measuring and recording equipment. The heat estimates so obtained are within 5 per cent of the measurements of a precision evaporimeter.
The water balance method is utilized relatively widely at present. According to our studies, soil moisture flux must be taken into account in those areas of the North China Plain with secondary salinization due to their higher water table. The error of estimating soil moisture flux by the water balance method is 4 per cent according to hydraulic soil evaporimeter measurements.
Evaporimeters have the longest history and are still an effective means of collecting long-term field evaporation data. Their records may also serve as a standard for testing other methods.
Crop water consumption data used in this paper are chiefly from 19601965 experiments in Dezhou, Shandong Province, and the outskirts of Beijing Municipality. Field water consumption estimates for the region are from empirical formula (3) mentioned later, which is obtained from the experimental data.
Field evaporation can be divided into two categories, that of irrigated fields and that of unirrigated fields. Irrigated field evaporation (E') refers to that with sufficient soil moisture for crop growth. It is given by the formula
E1 = a E0 (2)
where a is a water consumption coefficient reflecting the crop biology. Table 1 presents the value of a for some principal crops obtained from observations by the Institute of Geography on evaporation from irrigated fields and water surfaces during 1961-1965 in Derhou, Shandong Province, and during 1964-1965 in the rural areas of Beijing. The value of a for rice comes from a collection of papers.
Table 1. The Value a for Major Crops in the North China Plain
|Crop||Millet||Winter wheat||Summer maize||Paddy rice||Soybean|
The determination of water consumption in unirrigated fields is more complex because variations in soil moisture must also be taken into account in addition to differences in crop varieties and weather conditions.
According to data on the total water consumption of irrigated and unirrigated wheat, the ratio between the two is quite closely related to the amount of precipitation from September to May (Figure 2). With the current lack of measured data, there is still some practical significance to estimating the water consumption of winter wheat without irrigation. The method of calculating is to use
where Eu is the total water consumption during the entire growth period of unirrigated winter wheat, and P is precipitation.
FIELD WATER CONSUMPTION ESTIMATES
Equation (3) has been used to determine the water consumption of winter wheat, summer maize and winter wheat plus summer maize. The results are depicted in Figures 3, 4 and 5. It must be noted that these estimates are based on the assumptions that soil moisture is completely adequate, soil fertility is of medium or high grade and that crop yields are at least 2.25 t/ha.
From Figure 3 we can see that the water consumption of winter wheat is generally 400 to 550 mm/annum on the North China Plain. In the broad area north of the Huang He and between the Beijing-Guangzhou and Beijing-Pukou (Nanjing) rail lines, the value lies between 450 and 550 mm/annum. In most other areas it is 400 to 450 mm, and in small stretches along the coast it is less than 400 mm.
The distribution of summer maize water consumption in the region is similar to that of winter wheat. The range is between 300 and 400 mm/annum.
Total annual field water consumption, based on two crop seasons (winter wheat and summer maize), is approximately 700 to 850 mm. It exceeds 850 mm in the Heilonggang district, where it is at a maximum. It is 800 to 850 mm/annum in other areas north of the Huang He and it varies between 700 and 800 mm/annum in the Huai He basin.
Data on 1951-1977 winter wheat/summer maize water consumption (including bare land evaporation between crop rotations) from Yucheng County, Shandong Province show two peaks within the year (Figure 6). The first peak is in April and May, the driest period, when winter wheat is in the booting and milking stages and water needs and supply are extremely out of balance. The second peak is during July and August, the earing and milking periods for summer maize. This is when precipitation reaches a peak, so there is a slight surplus of moisture. Even if this excess portion does not form surface runoff, it can serve to supplement autumn moisture insufficiencies. From this we can see that the field moisture deficit in the North China Plain occurs mainly in the spring.
Figure 2. Correlation of E2/E and Precipitation for Winter Wheat
Figure 3. Distribution of Water Consumption of Winter Wheat under Irrigation
Figure 4. Distribution of Water Consumption of Summer Corn
Figure 5. Annual Field Water Consumption
Figure 6. Monthly Variation in Precipitation and Evaporation (average, 1951-1977 Yucheng county)
WATER CONSUMPTION IN SEVERAL OF THE MAIN CROPPING SYSTEMS IN THE CHINA PLAIN
Historically, the main cropping system in the North China Plain was three crops in two years. The chief cropping pattern was winter wheat-summer maize (or soybeans or millet)-spring millet (or spring maize, Chinese sorghum or sweet potatoes). In recent years, a double cropping system consisting in the main of winter wheat-summer maize has developed quite rapidly and has already become one of the main cropping systems.
In addition to our estimates, we have used data from several of our institute's experimental stations and experimental reports on water requirements from irrigation experiment stations to analyze the water consumption of the principal cropping systems. Our results are listed in Table 2.
From this table we can see that the long-run average annual water consumption of three crops in two years is the lowest, so this system is suited to the scarce and unevenly distributed precipitation in North China. In view of this, we must not overemphasize increasing the multiple cropping index in areas where irrigation conditions are inadequate but instead stress systems which are the most appropriate to the locality. Among the double cropping systems, the water consumption of winter wheat/summer maize is the lowest, so it is also relatively well suited to the precipitation conditions of North China. This system can make full use of the region's moisture resources as well as its light and heat.
Winter wheat occupies a significant position in all the cropping systems discussed above. It is therefore of great importance to study the patterns of winter wheat water consumption in order to understand field moisture conditions and even the field water balance in the North China Plain.
The growing period for winter wheat is very long, about 240 days. Only a small amount of precipitation falls during this period and this can only satisfy onethird of the total water requirement, which is quite high, fluctuating between 400 and 550 mm. Of this, evapotranspiration constitutes about 60 to 70 per cent.
It is about 180 days from sowing to greening up. During this period the crop remains in the seedling stage with small ground coverage, so soil evaporation between the plants predominates. Water consumption during this period constitutes about 30 to 40 per cent that of the entire growing season. There are only 60 days from jointing to harvest, hut water consumption is about 60 to 70 per cent of the total. The shortage of water during this period is even more clearly felt, and supplementary irrigation becomes necessary.
Table 2 Total Annual Water Consumption of Several Major Cropping Systems in the North China Plain
|Cropping system||Cropping panern||Total
|3 crops/2 years||Winter wheat??summer maize??millet||735||Water
consumption for the
three crops in two years
systems includes fallow
land evaporation during
the period from October to
|3 crops/2 years||Winter wheat??summer soya??millet||835|
|2 crops/year||Winter wheat??summer maize||840|
|2 crops/year||Winter wheat??soybean||1,050|
|2 crops/year||Winter wheat??rice||1,020|
THE ESTIMATION OF MOISTURE SURPLUS OR DEFICIT
The surplus or deficit of moisture ?is usually expressed by the difference between potential evaporation and precipitation, without consideration of runoff loss. That is,
D= E0 - P (4)
When E0 > P. there is a moisture deficit; when E0 < P. there is a moisture surplus.
Equation (4) can only approximately reflect the surplus or deficit of moisture in a region. In this chapter when we have estimated the amount of moisture surplus or deficit, we have mainly analyzed the moisture conditions of irrigated farmland and the growing period of winter wheat. A comparison of water consumption in irrigated fields and precipitation can reflect relatively accurately the degree of moisture surplus or deficit in this region and, in particular, may provide a reference base for the design of water transfer schemes.
In North China, the growing season for winter wheat is basically the dry portion of the year. Sowing is done after the rainy season of one year and the wheat is harvested before the next year's rains. Here we focus on analyzing moisture supply conditions during the growth period of winter wheat as this provides us with a general approximation of the water deficit for the entire year.
Figure 7 is a map showing the distribution of the difference between precipitation and field water consumption. The zero line, where the two elements balance over the course of a year, runs from Nanyang through Jining to the Shandong peninsula, coinciding with the 800 mm isohyet. South of this line there is surplus moisture; to the north is deficit.
In most areas north of the Huang He the field water deficit is 200 to 300 mm, and it exceeds 300 mm in the region of Xingtai, Nangong and Hejian. Except in a small stretch in the west, precipitation in the Huaibei Plain can satisfy the water requirements of the crops. Of course, these calculations are only long-term annual averages and do not take into account runoff losses or seasonal variations in precipitation.
We have here adopted three different methods of calculating moisture supply conditions during the growing period of winter wheat: (1) the difference between potential evaporation and precipitation (E0 - P); (2) the difference between the water consumption of winter wheat under irrigated conditions and precipitation (Ei - P); and (3) the difference between winter wheat water consumption under irrigated and unirrigated conditions (Ei - Eu). The precipitation data used here cover the period from September to the following May.
Figure 7. Differentiation Between Annual Precipitation and Annual Field Water Consumption
Table 3 presents a data analysis for 11 sites north of the Huang He. The water deficit ranges are 300 to 380 mm by the first method; 250 to 330 mm by the second; and 140 to 170 mm by the third. It is our opinion that the results obtained by the first two calculations are too large. The first method uses potential evaporation, which is about 10 per cent larger than field water consumption, so it clearly cannot represent the actual water consumption of winter wheat. The drawback of the second method is that it does not consider the moisture stored in the soil or the groundwater recharge into the soil water. The third method reflects the difference between water consumption when soil moisture is adequate and winter wheat water consumption under natural water conditions. The latter takes soil moisture supply conditions into account and its results are relatively closer to the actual water deficit.
Table 3 Moisture Surplus and Deficit in the North China Plain During the Winter Wheat Growing Period (in mm)
|Eo - P||380||362||309||330||374||346||348||347||360||318||350|
|Ei - P||327||304||259||277||320||295||295||294||308||268||299|
|Ei - Eu||167||163||144||148||173||161 *||156||164||163||149||153|
Key: Ei: water consumption
under irrigated conditions;
Eu: water consumption under unirrigated conditions;
Eo potential evaporation;
P: precipitation from September to May.
Our estimates of potential evaporation and field water consumption show that at the present state of agricultural technology, natural moisture conditions fail to satisfy agricultural requirements. A 100 to 300 mm deficit in moisture is felt throughout the area north of the Huang He. Seasonally, spring drought is the main threat. Therefore, if we are to transfer water, the area north of the Huang He should be the key water importing region with the main purpose being to solve the problem of spring drought.
Cheng Tianwen and Cheng Weixin, 1980, "Measurement and estimation of field evaporation and potential evaporation", Collected Geographical Papers, Science Press, Beijing.
Gao Guodong et al., 1978, "Estimation and distribution of maximum potential evaporation in China", Acta Geographica Sinica, Vol. 33, No. 2.
Lu Qiyao et al., 1965, "A study of wet and dry periods in China and a delineation of wet and dry regions", Acta Geographica Sinica, Vol. 31, No. 1.
Qian Jiliang and Lin Zhiguang, 1965, "A preliminary study of dry and wet climatic regionalization in China", Acta Geographica Sinica. Vol. 31, No. 2.
Zhu Gangkun and Yang Renzhang, 1955, "The application of meteorological records in economic construction, part II: A preliminary study of evaporation in various parts of China", Journal of Meteorology, Vol. 26, Nos. 1-2.
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