An overview on observational instruments and measuring methods for dewfall

Qiang Zhang , XiaoCui Hao , XiaoMei Wen , Sheng Wang , Jian Zeng ,3

1. Institute of Arid Meteorology, China Meteorological Administration; Key Laboratory of Arid Climatic Change and Reducing Disaster of Gansu Province; Key Open Laboratory of Arid Climatic Change and Disaster Reduction of China Meteorological Administration, Lanzhou, Gansu 730020, China

2. College of Atmospheric Sciences, Lanzhou University, Lanzhou, Gansu 730000, China

3. Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing 100081, China

*Correspondence to: Qiang Zhang, Institute of Arid Meteorology, CMA; Key Laboratory of Arid Climatic Change and Reducing Disaster of Gansu Province; Key Open Laboratory of Arid Climatic Change and Disaster Reduction of CMA, Lanzhou 730020,China. Email: Zhangqiang@cma.gov.cn

An overview on observational instruments and measuring methods for dewfall

Qiang Zhang1,2,3*, XiaoCui Hao1,2, XiaoMei Wen1, Sheng Wang1, Jian Zeng1,3

1. Institute of Arid Meteorology, China Meteorological Administration; Key Laboratory of Arid Climatic Change and Reducing Disaster of Gansu Province; Key Open Laboratory of Arid Climatic Change and Disaster Reduction of China Meteorological Administration, Lanzhou, Gansu 730020, China

2. College of Atmospheric Sciences, Lanzhou University, Lanzhou, Gansu 730000, China

3. Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing 100081, China

*Correspondence to: Qiang Zhang, Institute of Arid Meteorology, CMA; Key Laboratory of Arid Climatic Change and Reducing Disaster of Gansu Province; Key Open Laboratory of Arid Climatic Change and Disaster Reduction of CMA, Lanzhou 730020,China. Email: Zhangqiang@cma.gov.cn

This paper systematically summarizes previous measuring methods and observational instruments for the magnitude of dewfall on land surface, analyzes the characteristics of common observational instruments for land surface dewfall, and describes several basic dewfall measurement methods. Moreover, the basic principles of these methods and instruments are interpreted, and their advantages, disadvantages, and applicability are analyzed. Recommendations for the further improvement of these observational instruments and the development of dewfall measuring methods are presented, and new technologies and scientific proposals for exploiting dewfall are elucidated.

dewfall; measuring methods; observational instrument; exploitation and utilization technology

1. Introduction

Whenever the temperature of the land surface drops below the dew point temperature, atmospheric water vapor will condense into drops on the ground or on the surface coverings, which is commonly referred to as dewfall on land surface (Zhou, 1997). Dewfall on land surface is an important component of land surface water balance (Zhanget al.,2010) because it evaporates directly or is absorbed by shallow soil, and it can produce significant ecological and climatic effects. Human beings have exploited dewfall since ancient times. In ancient China, monks and priests living in the mountains used natural dewfall to steep tea and cook soup. In Europe, as early as the 6th century BC, the people of Feodosia in the Crimea began to gather dewfall to meet their water needs, and in 1912 it was reported that Russian rangers could collect 300 to 360 liters of dewfall with simple equipment (Zibold, 1995). In recent decades, the dry areas of France, Peru, Israel, Germany, and other countries have begun to develop technology for exploiting dewfall on land surface. Early in the 19th century, Wells (1814) produced a scientific exposition on dewfall, and in the mid-20th century the dewfall on land surface was explained even more scientifically (Monteith, 1957).

In nature, dewfall is relatively common and sometimes is considerable. For example, in China the total amount of dewfall on canopy at night in the dry season of the Xishuangbanna forest measured by Liuet al. (1998) was 1.36 mm, and from this they inferred that the cumulative dewfall in an entire year reached about 400 mm. In arid and semi-arid areas, dewfall is particularly important because of lack of surface precipitation. It plays a key role in maintaining the ecological balance and vegetation in such regions.Based on water balance theory, Chen (2002) initially calcu-lated that in the northwest arid area of Minqin, where the average annual rainfall is only 100 mm, the dewfall received by the surface soil ranges from roughly 100 to 200 mm,exceeding the contribution of the natural precipitation. Subsequent studies have demonstrated that dewfall is an important water source for maintaining vegetation in dry lands(Zhang and Huang, 2004; Zhang and Wang, 2007). In addition, many studies have shown that there is a considerable amount of dewfall in many parts of arid and semi-arid areas(Yaoet al., 2003; Zeng and You, 2008; Wanget al., 2009;Wenet al., 2009). In recent years, with the global drought increasing, the exploitation of dewfall on land surface has become a hot international topic.

Dewfall research has long been an important scientific issue in meteorology, hydrology, agriculture, ecology, and other related subjects (Zhao, 2006). Existing research has not only analyzed the physical characteristics of dewfall and its ecological and climate effects (Monteith, 1957; Duvdevani, 1964; Oke, 1987; Xuet al., 1994; Beysens, 1995;Li, 2002; Zhang and Wei, 2003; Yanet al., 2004), but has also discussed its influence on the water cycle on land surface layers and in observations (Zhanget al., 1996; Zhang and Zhao, 1998; An and Lv, 2004; Wang and Zhang, 2004;Zhang Het al., 2008). This has gradually led to basic understandings of the features and functions of dewfall, but there are still many technical problems associated with the accurate measurement of dewfall (Angus, 1958; Muselliet al.,2002). The lack of objective and real observations of dewfall on land surface seriously restricts the further understanding of formation and variation of dewfall as well as its eco-climatic effects.

Therefore, in recent years many scientists have sought to improve the measurement of dewfall on land surface and the accuracy of observation instruments. This has led to the development of some measurement methods having a theoretical basis (Muselliet al., 2002), and a number of practical measuring instruments have been proposed (Takenakaet al.,2003). Currently, various measurement methods of dewfall on land surface are in use but few are completely convincing(Zangvil, 1996) and there are few instruments accepted in the national observation either (Beysenset al., 2005).

For these reasons, this study compares the current measuring methods and observation equipment for dewfall on land surface, and analyzes their technical features and functions. We identify their key problems and shortcomings, and offer science-based recommendations for further improvement of dewfall measurement methods and technology(Zhang and Wang, 2008; Zhanget al., 2009) in order to exploit dewfall on land surface more effectively.

2. The dewfall observational instruments

Dewfall occurs on land surfaces when the atmospheric humidity of the surface layer increases or the temperature decreases. When the air temperature directly related to the land surface or surface features falls below the dew point,water vapor in the atmosphere condenses into water drops and attaches to the ground or surface coverings. The hydrologic state of the atmosphere and the land surface features are the most critical factors that affect the formation of dewfall on land surface. Therefore, accurate observation of dewfall on land surface involves measuring the amount of dewfall attached to the ground or land surface coverings,and its variation with natural atmosphere states and land surface features. Technically, this requires that three key conditions be met: (1) the measuring instruments must be capable of high accuracy as well as automatic recording so as to measure the dewfall accurately and record the process of dewfall in real time; (2) the land surface must be free from human disturbance and be in as natural a state as possible; and (3) the instruments cannot be oversized or too complicated so as to avoid influencing the natural state of the atmosphere. The dewfall measuring instruments in current use throughout the world can be generally classified into eight categories as follows (Pedro and Gillespie, 1982):container-type drosometer, flat drosometer, absorbent paper-type drosometer, Hiltner balance drosometer, lysimeter,electronic conductivity of soil moisture probe, microwave radiometer, and water balance calculation system.

2.1. Container-type drosometer

A container-type drosometer is a container having a metric or other scale that is designed to collect dewfall. Early in 1912 the Russian forester Zibold invented a primitive container-type drosometer (Zibold, 1995), that was a funnel-shaped container which had piled pebbles in the top.From 1928 to 1957, the hydrologists Chaptal (1932) and Jumikis (1965) improved this device and developed a "Zibold-type" drosometer. Recently the Chinese scientist Liu Wenjie and colleagues used a similar drosometer made of bamboo to measure the dewfall on canopy during the dry season in the Xishuangbanna tropical rainforest (Liuet al.,2001). Over the past 50 years or so, physicists and hydrologists have developed more scientific container-type drosometers. Most of these devices use a jar to collect dewfall; the jar has a scale on it or is weighable, and the collector, which is wide-mouthed and shallow-bodied in shape, is made from a material whose texture is as close to the natural surface or surface features as possible. The Kessler-Fuess dewfall recorder is typical among these (Noffsinge, 1965), and it uses an upward-opening aluminum cone as a dewfall collector.Tick marks on the cone are converted into weight units. Although it can simultaneously measure dewfall and its change,measurements made by this kind of drosometer are generally far from accurate due to its uneven shape. To a large extent, such measurements are of only climatological significance.

2.2. Flat drosometer

In order to overcome the structural defect (unevenness)of container-type drosometers, flat drosometers have been developed, the shape of which is closer to natural surfaces.Generally, a flat drosometer collects dewfall from a condensation surface such as a plate made of metal, glass,wood, or other materials that is placed horizontally at a certain height above the ground (Liuet al., 2001). The Duvdevani-type drosometer is one of the earliest flat drosometers (Duvdevani, 1947). It uses a special smooth-painted wood plate as a dewfall condensation surface, which is placed 1 m above the ground. However, this kind of drosometer is affected by the thermal properties and radiation characteristics of wood, which results in a gap between the measurement of dewfall and the natural surface. Its measurements have practical significance only in the case of comparative analysis of dewfall on a regional scale or larger.

More recently, to reduce the influence of flat surface heat conduction and radiation as much as possible, many scientists have been working to develop condensation plate materials that have heat conduction and radiation properties more closely resembling those of natural land surfaces. Takenakaet al.(2003) compared the amount of dewfall measured on various dewfall condensation surfaces that were made of polytetrafluoroethylene (PTFE), heat-resistant glass, stainless steel, aluminum, and other materials, and found that the dewfall collected varied widely on these surfaces.

To exclude measurement interference from heat conduction and radiation properties of condensation surfaces, the French physicist Beysens and colleagues (Monteith, 1957)developed a flat drosometer with a relatively complex but more scientific condensation plate: its upper layer is a Plexiglas sheet whose size is 400×400 mm and thickness is 5 mm;the middle layer is 12.5-μm-thick aluminum foil; and the bottom is an adiabatic polystyrene foam. According to comparative studies of dewfall measured by this device in Corsica, Bordeaux, Ayaqiu of Gelenuobo, and elsewhere in France, the device is relatively reliable. However, in general,a flat drosometer cannot calculate the rate of dew formation effectively, and it is rather difficult to measure the dewfall accurately with it.

2.3. Absorbent paper-type drosometer

In the case of heavy dewfall, flat drosometers cannot accurately collect and measure dewfall. To address this problem, absorbent paper-type drosometers use absorbent paper as the dewfall collector, which has the advantages of convenience in collecting and weighing dewfall, and thermal radiation and hydrophilic properties that are close to those of vegetative leaves. Moreover, the low heat capacity of absorbent paper makes it easy to reach thermal equilibrium. Luo and Goudiaan (2000a) used blotting paper as a condensation plane to successfully measure the dewfall in rice fields at night. Barradas and Glez-Medellin (1999) used filter paper discs as a condensation plane to measure the dewfall of coastal forests in Mexico and comparatively analyze the difference of dewfall collected by filter paper and green leaves. They found that the dewfall collected by filter paper was just 5% less than that of leaves, which basically falls within the acceptable range of error. Yanet al.(2004) used four condensation surfaces (filter paper, poplar wood sticks,sunflower stalks, and glass) to research the dewfall in swamps in the Sanjiang Plain; they found that poplar wood sticks functioned better than the other three condensation surfaces in that context.

2.4. Hiltner balance drosometer

The basic principle of the Hiltner balance drosometer is that it can continuously weigh a dew condensation plate mounted at a height of 2 m above the ground, enabling it to measure the changes of dewfall on the plate (Chaptal, 1932;Duvdevani, 1947; Noffsinge, 1965; Pedro and Gillespie,1982; Barradas and Glez-Medellin, 1999; Luo and Goudriaan, 2000a; Liuet al., 2001; Ninari and Berliner,2002). However, the obvious design flaws in this device are:(1) the device and the surface soil are isolated by air because it hangs above the ground surface; (2) the properties of its plastic condensation plate are obviously different from those of soil; and (3) in nature, some of the dewfall on the soil surface will seep into the soil, whereas the dewfall on this condensation plate will remain accumulated on the plate.These defects lead to considerable differences in the energy balance of hanging an artificial condensation plane above the soil surface. Thus, we generally believe that a Hiltner balance drosometer actually measures potential dewfall, and the amount of dewfall measured is mainly controlled by local weather conditions. In fact, to a certain degree, other types of drosometers also have similar problems.

2.5. Lysimeters

In contrast, lysimeters take dewfall measurements under the conditions closest to the natural state (Jacobset al.,2000). With the recent improvements of electronic and information technology, they can utilize automated, digital,and remote sensing technology. The core part of a lysimeter is the precise electronic scale buried in the soil. In order to minimize the marginal effects of a lysimeter, the built-in pan should be as large as possible and soil in the pan should be separated from external part in the horizontal direction to prevent the effects of horizontal exchange of soil moisture. Through experimental studies, Rosenberg(1973) suggested that a lysimeter is the best device to measure dewfall because it not only maintains the natural state of the condensation surface, but also rules out the effects of nearby plants "spitting water" on the measurement of the dewfall. However, measurements of dewfall in the Negev Desert in Israel by Horrold (1951)et al. with miniature evaporation devices have been questioned(Monteith and Unsworth, 1991); this is possibly attributable to the small pan scales of this type of evaporation measurement device. In China, Xuet al. (1994) measured and analyzed the dewfall in wheat fields using a similar instrument.

2.6. Electronic conductivity measured with a soil-moisture probe

These examples demonstrate that the traditional dewfall measuring instruments that use dewfall collectors cannot completely avoid the changes of thermal and dynamic properties of the condensation land surface. With the development of electronic and material technologies, however, a soil-water vapor meter based on electronic conduction began to appear (Bunnenberg and Kuhn, 1980). It utilizes the changes in the sensor’s thermal conductivity to measure the dewfall on the soil surface. However, it also causes interference in the accurate measurement of the dewfall because the thermal conductivity in the measurement process affects the process of condensation and evaporation to some extent.

2.7. Microwave radiometer

Currently, the international community is still working on remote sensing methods of measuring dewfall. For example, experimental studies by Wigneronet al. (1996) suggested that the signal of dewfall on the ground surface can be observed by a passive microwave radiometer. Studies by Ridleyet al. (1996) and Jackson and Moy (1999) found that a microwave radar mounted on a satellite can sense dewfall.In the future, ground-placed or space-borne microwave radiometers that have significant technical advantages in the investigation of the spatial distribution of dewfall may be increasingly used for practical measurement of dewfall on ground surfaces.

2.8. Water balance dewfall calculation systems

Some scholars observe dewfall on the basis of land surface water balance. This method does not utilize an instrument per se; rather, this is accomplished by using data from a set of observation systems consisting of energy balance and precipitation measurement instruments. Its core idea is that the amount of dewfall can be estimated based on the principle of water balance, using actual observed evaporation and rainfall measurements. For example, Jacobset al.(2000), following Pedro and Gillespie(1982), made an estimation of the dewfall in the Negev Desert using the eddy correlation method; their results were in good agreement with measurements by micro-evaporation. However, this dewfall observation system is only an indirect estimation of dewfall. Its possible defect lies in the fact that there may be some contribution of uncertain water. Also, a series of several short-term observations could possibly introduce observation errors; in the case of small dewfall amounts, the actual measurement could be overshadowed by the measurement errors. Consequently, this method has significant limitations.

Theoretically, evaporation and precipitation can also be calculated by using the land-atmosphere coupled numerical model, which does not incorporate direct observation data,and then the amount of dewfall can be estimated according to the principle of land surface water balance. For example,Wilsonet al. (1999) used the soil-canopy-atmosphere numerical model to estimate the dewfall on potato leaf surfaces at different vertical heights. Beysenset al. (2005) used the land-air quality exchange numerical model to simulate the variation of dewfall on land surfaces during daytime. However, the estimation of dewfall by numerical modeling is not sufficiently reliable because the current numerical models do not simulate the evaporation and precipitation successfully.

Another approach is to estimate dewfall on land surface through the development of a statistical model that links dewfall, certain key climatic factors, and land surface physical quantity. For example, Luo and Goudriaan (2000b)simulated the dew water on rice canopy by using a statistical model of micro-climate, and assessed the appearance time of dewfall. Madeiraet al. (2002) established a semi-empirical relationship model between dewfall and thickness and height of clouds based on energy balance, and estimated the variation of dew water quantity. However, these empirical statistical models and semi-empirical statistical models generally lack a comparatively solid theoretical basis, and the relationships used in these models vary with time and place,leading to few ideal dewfall estimates.

3. The dewfall measuring methods

3.1. The measuring methods

Currently, most of the instruments used for dewfall measurement rely on volume or weight measurements, or on new technology such as electronic sensing and remote sensing. Dewfall measuring methods can be classified into six representative types according to the measuring principle and characteristics of the instruments.

(1) Dewfall collection method. The principle of this method is to use an artificial condensation surface to collect the dewfall and measure the collected dewfall by volume or weight. The artificial condensation surface can be a container, a flat plate, or absorbent paper. A container drosometer can measure the dewfall directly, whereas flat plate and absorbent paper drosometers only measure the dewfall indirectly.

(2) Direct weighing method. This method can be applied to measure the variation of dewfall directly, using a condensation plate hanging balance or a lysimeter with its major component (a large electronic scale) buried. A condensation plate hanging balance is an artificial condensation surface that does not touch the land surface, whereas a lysimeter consisting of an electronic scale condensation plate contacts the land surface directly.

(3) The balance calculation method. This method estimates the value of dewfall based on the principles of land surface water balance using the measurement of evaporation,precipitation, and other water components.

(4) The electronic conduction method. This method measures dewfall by the sensitivity of the sensor’s heat transfer capacity to dewfall.

(5) The remote sensing method. The basic principle of this method is that the signals received or launched by an active or passive microwave radiometer and other remote sensing instruments are sensitive to the variation of dewfall on the land surface. Thus, high sensitivity is required for accurate measurement.

(6) Soil moisture inversion method. According to the measurements of soil moisture at different depths, dewfall is reckoned by analyzing the variation of soil moisture.

3.2. Comparison of measuring methods

All of these dewfall measuring methods have merits and demerits in their performances and have different practical applicabilities. Generally speaking, the dewfall collection methods are simple and cheap to implement, but their dewfall measurements vary greatly from actual dewfall values due to their non-natural condensation surfaces. The direct weight method is somewhat superior to the other methods,especially lysimeters consisting of buried large electronic scales whose condensation surfaces maintain the natural properties of the land surface. However, this method is expensive and requires highly accurate electronic scales. Also,marginal effects of changes of the surface environment and soil structure caused by the weighing device can influence the measurement results. The process of heat conduction in the electronic conduction method may interfere with the condensation process of water vapor, which introduces some doubts as to its accuracy, although this method avoids the change of natural land surface that occurs in dewfall collection methods. The balance calculation method is an indirect measurement. Its main problem is that the results are greatly affected by various errors in the observation of water balance components, and thus the reliability of this method is weak. The soil moisture inversion method is not only limited by the accuracy of soil moisture measurement and the inhomogeneity of soil moisture, but is also disturbed by vertical transport and level exchange of soil moisture. Therefore,this method is usually used as a reference and its results should not be used directly.

In summary, the dewfall collection method can be used in cases where high accuracy is not required. The balance calculation method can be used in cases of heavy dewfall.The soil moisture inversion method is currently unrealistic for measuring actual dewfall. The electronic conduction method must consider systemic errors caused by certain interferences that require objective corrections. Lysimeters used in the direct weighing method may be appropriate for quantitative and continuous observation of dewfall.

4. The exploitation and utilization of dewfall

Artificial precipitation enhancement and preservation of soil moisture have become the primary focus of the development and exploitation of cloud water and soil moisture.However, only a few tentative experiments on the development and exploitation of dewfall have been carried out to date; such development and exploitation has not always been well regarded (Zibold, 1995). Nevertheless, domestic and international research (e.g., Luo and Goudriaan, 2000a)has shown that the amount of dewfall in nature is considerable and that dewfall has great exploitation value.

Dewfall has many desirable eco-climate features compared with natural precipitation (Leyton, 1985). First, the direct absorption of dewfall adhered on the leaves and blades of vegetation can decrease the internal water loss,which in some cases is more timely and effective than water absorption by roots. Second, dewfall is more evenly distributed over time than rainfall and does not cause oversaturation and soil erosion, and thus can be fully absorbed by soil or vegetation. Third, the efficiency of dewfall is much higher than normal precipitation because dewfall is more easily absorbed by plant roots. Fourth, with longer durations and higher frequencies than normal precipitation, dewfall plays a key role in the case of extreme loss of plant water. Finally,dewfall is an important water supply for natural ecosystems biological soil skin, insects, and small animals, and thus has a beneficial effect on the stabilization of sand in desert areas(Agam and Berliner, 2006).

The amount and duration of dewfall are affected by a variety of factors, such as local climate, features of the land surface, and so on. Improvement of the local climate conditions and adaption of the features of land surface can create more suitable weather conditions and surface features for the formation of the dewfall, and even increase the volume of dewfall. In the practice of exploiting dewfall, selecting favorable weather conditions and designing favorable condensation surfaces are of great importance.

In terms of local climate conditions, the following conditions are favorable for water vapor condensation: clear sky or few clouds, high relative humidity, moderate wind speed,strong inversion of temperature from the ground to the atmosphere, and low surface temperature. Tests have determined that air relative humidity greater than 90%, wind speed of 1 to 2 m/s, surface temperature below the dew point,and a relatively strong gradient of atmospheric inversion all favor the formation of dewfall. In terms of geographical conditions, flat, open terrain is most favorable to facilitate smooth air flow and cool the land surface. Also, temperature and humidity gradients are large near land surfaces and thus there are considerable differences in the cooling effects of condensation surfaces and water vapor conditions, which results in difference in the volume of dewfall. Gioraet al.(2002) placed dewfall collectors at different heights in the Negev Desert in Israel found that better results of dewfall collection existed at certain heights.

Under the ideal conditions described above, the volume of dewfall can reach more than 0.5 mm per day, whereas it could be an order of magnitude smaller in volume (or even no dewfall would be produced) under relatively poor conditions. Thus, the climate and geographical conditions significantly influence the volume of natural dewfall.

Under the same local climatic and environmental circumstances, both an increase in the albedo and a reduction in thermal conductivity of the condensation surface are likely to increase the amount of dewfall. Therefore, the development and innovation of condensation plane design technology, which is more effective than the creation of favorable local climatic and environmental conditions, is the main direction of current dewfall exploitation.

Dewfall exploitation is therefore mainly achieved by the improvement of the condensation surface. The most common improvement is to produce dewfall collectors that are relatively wide open and small in size; this kind of collector can collect more dewfall than that under the natural states(Nikolayevet al., 1996). A wide-open and small collector,on one hand, maximizes the intensity of surface radiation cooling and reduces the thermal inertia of the condensation surface at night, which effectively lowers the surface temperature and extends the time when the temperature of the condensation surface is below the dew point. On the other hand, it can make the surface water vapor more fully exchange. This type of dewfall collector includes the representative upside-down shallow cone and panels.

Secondly, improvement of the properties of thermal insulation and reflection of the condensation surface is commonly considered in the design of a dewfall collector. This improvement can significantly increase the cooling rate of the condensation surface and the efficiency of dewfall formation. Generally, materials of low thermal conductivity and high albedo, or compound materials of good thermal insulation, are selected in the manufacture of condensation surfaces. Currently, the application of compound materials is relatively common due to their good thermal insulation and high albedo.

Thirdly, hydrophilic materials can also be considered to construct the condensation surface and increase the formation of dewfall. The reason is that the more hydrophilic the condensation surface is, the smaller the wetting contact angle is and the higher the dew nucleation rate and growth rate are, and thus the greater the volume of dew water formed.Generally, polyethylene (PE) and other inorganic materials have good hydrophilicity and are more commonly used in the practice. A clean and smooth condensation surface made of inorganic materials is best in terms of hydrophilicity, and its wetting contact angle should be close to 0 degrees. Conversely, a rough condensation surface covered with a hydrophobic activated membrane is not prone to producing dewfall because its wetting contact angle can be as high as 180 degrees.

Fourthly, the inclination of the condensation surface can be taken into consideration (without regard to the thermal radiation and hydrophilia of the material) to effectively increase the dewfall. Experiments were designed by Muselliet al. (2002) for different angles between the condensation surface and the horizontal plane, and 30 degrees was found to be the angle that produced the maximum volume of dew collection. These experiments fully considered the reflectivity of flat, gravity, and many other effects.

Fifthly, improvement of the structural design of the condensation surface of the dew collector also helps to increase the volume of collected dewfall. Tests have shown that a netted structure is the best and most efficient structural design for the eradicative cooling and condensation area(Zhang Qet al., 2008). Moreover, the condensation surface of a netted structure also intercepts fog to a certain extent.Currently, more experiments are underway to try to combine these superior technologies mentioned above, including selecting materials with better radiation cooling effect and strong hydrophilia, mesh structure, and the optimal placement of the condensation surface, to develop highly technical dew collectors. Dew collectors consisting of mesh metal condensation surfaces have been shown to have very good effects on the enhancement of collected dewfall. In order to effectively increase the volume of dew collection, a small metal can can be put inside a large metal one, or thin metal wire can be woven into a pine needle shape on the net to enhance the radiation cooling and increase the condensation surface area. Both the size of the metal mesh and the amount of wire "pine needles" can be changed freely, and the structure can be adjusted according to the air moisture and wind speed.

Recently, some efforts have been made to improve dew collection by using new materials. For example, Zhang Qet al.(2008) have been using non-woven mesh made of nano-nylon as the condensation surface of a dewfall collector.

5. Conclusions

The volume of dewfall on land surfaces depends not only on the local weather conditions, but is also affected by the physical characteristics of the land surface and the radiation, thermal, and dynamic properties of the surrounding environment, which greatly increases the difficulty of accurately measuring dewfall on land surfaces. Dewfall observation instruments in current use include container-type drosometers, flat drosometers, absorbent paper-type drosometers, Hiltner balance drosometers, and lysimeters, which are all direct measurement methods (note that a lysimeter does not have a man-made collector and its condensation surface is the natural surface, which makes it different from the other instruments). Electronic conductivity soil-moisture probes, microwave radiometers, and water balance dewfall estimation systems are all indirect measurement methods and require signal conversion or conversion of physical quantities.

The direct measurement methods using dewfall collectors change the properties of the condensation surface, and thus their measurements, to some degree, are not objective.The indirect measurement methods, including electronic conductivity soil-moisture probes and microwave radiometers, often lack accuracy because of the limitations and requirements of the electronics and microwave technology.The water balance dewfall estimation system is influenced significantly by observation error from the water components, and in many cases the actual volume of dewfall is masked by measurements errors. Comparatively, the lysimeter is an ideal dewfall measuring device at present and can basically measure the volume of dewfall with tolerable errors. There still may be too large a gap between simulated and actual dewfall on land surfaces to be useful for practical dewfall estimation, although, theoretically, numerical modeling or statistical models can also be used to estimate the volume of dewfall.

Currently, there is no internationally uniform dewfall measurement method. In the future, a comprehensive approach that includes many different methods may be taken to measure the volume of dewfall on land surfaces. Given the current technical level and the technical requirements of accurate dewfall measurement, it is a good choice to use a large lysimeter as the primary instrument to measure dewfall,combined with the observation of water vapor flux near the ground by the eddy correlation method and measurements of soil moisture by TDR (time domain reflectometry) or TDT(time domain transmissometry), based on whether the actual volume of dewfall on the land surface is determined and identified synthetically.

As the global water shortage has become increasingly evident, the development and exploitation of dewfall has become more urgent and many technical measures have been developed to effectively enhance the formation of dewfall on land surface. Given the new materials and new technologies now being used, especially with the maturation of new dewfall collectors with netted structures placed above the ground and made of highly hydrophilic material with good radiation effects, the exploitation of dewfall will achieve large-scale and industrialized development.

This study was supported by the National Science Foundation of China (Grant Nos. 40830957 and 40575006).

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10.3724/SP.J.1226.2011.00078

12 September 2010 Accepted: 19 November 2010