DEVELOPMENT OF A LYSIMETER TO MEASURE THE EVAPOTRANSPIRATION OF A PLANT
Large weighable lysimeters allow a precise determination of the soil water balance and the quantification of both water exchange at the soil-atmosphere interface and the flux below the root zone toward the groundwater. If well embedded into an equally-vegetated environment, they reach a hitherto unprecedented accuracy in estimating precipitation (P) by rain, dew, fog, rime and snow, and evapotranspiration (ET). Lysimeters largely avoid errors made by traditional measurement systems, such as the wind error of Hellmann rain gauges, the island error of class-A pans, or errors from soil-water measurements that are subject to subsurface heterogeneity. If the amount of seepage water is added to the lysimeter mass, temporal changes of the lysimeter mass can be used to solve the water balance equation for atmospheric fluxes. Increasing mass indicates P, decreasing mass ET. The determination of the net water balance (sum of P and ET) is accurate and robust. A problem arises in the separate estimation of the underlying P and ET fluxes, because weight differences in specified time intervals are affected by stochastic fluctuations due to mechani- cal vibration, which may be caused by wind or other factors.
CHAPTER ONE
1.0 INTRODUCTION
Lysimeter is a measuring device which can be used to measure the amount of actual evapotranspiration which is released by plants, usually crops or trees. By recording the amount of precipitation that an area receives and the amount lost through the soil, the amount of water lost to evapotranspiration can be calculated. Lysimeters are of two types: Weighing and non-weighing.
Most weather stations record rainfall but few measure evaporation despite it being an important parameter in climatology. What happens to rainfall once it has reached the ground also is of interest to ecologists, hydrologists and water engineers. With modern automatic instrumentation it is relatively straightforward to calculate evaporation and is done so by professional hydrologists and others. Some weather enthusiasts use the Piche evaporimeter exposed within their thermometer screen to measure evaporation. Evaporation can be measured, at a site which also measures rainfall, by using a lysimeter which gives additional information on soil water balance.
1.1 OBJECTIVE/AIM OF THE STUDY
The aim of this study is to evaluate algorithms that aim on eliminating the effects of these fluctuations and to estimate actual fluxes across the soil-atmosphere boundary and the soil water balance from lysimeter measurements. We use synthetic and real measured data from large lysimeters to test which strategies of data evaluation can be applied, and which degree of accuracy can be reached.
1.3 SIGNIFICANCE OF THE PROJECT
A lysimeter is most accurate when vegetation is grown in a large soil tank which allows the rainfall input and water lost through the soil to be easily calculated. The amount of water lost by evapotranspiration can be worked out by calculating the difference between the weight before and after the precipitation input.
For trees, lysimeters can be expensive and are a poor representation of conditions outside of a laboratory as it would be impossible to use a lysimeter to calculate the water balance for a whole forest. But for farm crops, it can represent field conditions well since it is done outside the laboratory. A weighing lysimeter, for example, reveals the amount of water crops use by constantly weighing a huge block of soil in a field to detect losses of soil moisture.
1.4 SCOPE OF THE STUDY
With the current interest in global climate change is it not a good time to increase the number of observations of evaporation, particularly by amateurs, weather enthusiasts and in schools? In recent issues of the Bulletin of the Climatological Observers Link there are only 2 or 3 observations from evaporation tanks and 5 or 6 from Piche evaporimeters in over 300 reporting stations. I suspect that lack of interest is not a factor, but is one of practicability or cost of equipment. Evaporation tanks or pans are expensive, even if space was available at the average weather station.
1.5 TYPES OF LYSIMETERS
There are two types of lysimeters, they are as follow:
– Weighing Type
– Non-weighing Type
1.6 LIMITATIONS OF LYSIMETERS
– Reproduction of physical conditions in field (say, temperature, water table position, soil texture, density etc)
Weather parameters, crop characteristics, management and environmental aspects are factors affecting evaporation and transpiration. The related ET concepts presented in Figure 3 are discussed in the section on evapotranspiration concepts.
Weather parameters
The principal weather parameters affecting evapotranspiration are radiation, air temperature, humidity and wind speed. Several procedures have been developed to assess the evaporation rate from these parameters. The evaporation power of the atmosphere is expressed by the reference crop evapotranspiration (ETo). The reference crop evapotranspiration represents the evapotranspiration from a standardized vegetated surface. The ETo is described in detail later in this Chapter and in Chapters 2 and 4.
Crop factors
The crop type, variety and development stage should be considered when assessing the evapotranspiration from crops grown in large, well-managed fields. Differences in resistance to transpiration, crop height, crop roughness, reflection, ground cover and crop rooting characteristics result in different ET levels in different types of crops under identical environmental conditions. Crop evapotranspiration under standard conditions (ETc) refers to the evaporating demand from crops that are grown in large fields under optimum soil water, excellent management and environmental conditions, and achieve full production under the given climatic conditions.
Management and environmental conditions
Factors such as soil salinity, poor land fertility, limited application of fertilizers, the presence of hard or impenetrable soil horizons, the absence of control of diseases and pests and poor soil management may limit the crop development and reduce the evapotranspiration. Other factors to be considered when assessing ET are ground cover, plant density and the soil water content. The effect of soil water content on ET is conditioned primarily by the magnitude of the water deficit and the type of soil. On the other hand, too much water will result in waterlogging which might damage the root and limit root water uptake by inhibiting respiration.
When assessing the ET rate, additional consideration should be given to the range of management practices that act on the climatic and crop factors affecting the ET process. Cultivation practices and the type of irrigation method can alter the microclimate, affect the crop characteristics or affect the wetting of the soil and crop surface. A windbreak reduces wind velocities and decreases the ET rate of the field directly beyond the barrier. The effect can be significant especially in windy, warm and dry conditions although evapotranspiration from the trees themselves may offset any reduction in the field. Soil evaporation in a young orchard, where trees are widely spaced, can be reduced by using a well-designed drip or trickle irrigation system. The drippers apply water directly to the soil near trees, thereby leaving the major part of the soil surface dry, and limiting the evaporation losses. The use of mulches, especially when the crop is small, is another way of substantially reducing soil evaporation. Anti-transpirants, such as stomata-closing, film-forming or reflecting material, reduce the water losses from the crop and hence the transpiration rate.
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