AUTOMATED

M E A S U R E M E N T S OF T H E R M A L AND WATER

BALANCE E L E M E N T S Yu. E. G i r s h o v i c h , M. A. K a g a n o v , I. G. M u s h k i n , Yu, A. P e s c h a n s k i i , a n d M. H. Y a k k e r

UDC 631.4:53.08

In carrying out wide-scale investigations in agricukure, melioration, and geophysics it becomes necessary to evaluate thermal and water balance elements. These measurements acquire special significance in connection with the programming of agricultural crop yields. Thermal balance investigations are aimed at discovering the optimum plant development conditions and their relationship to various controlling factors which may be considered to include technological factors (sowing density, structure of the cover crop, water and fertilizer supply conditions, intermediate cultivation processes) and external effects (meterological conditions, particular features of graded crops, etc.). Practical application of data on thermal balances, and in particular on the total evaporation of a field, can be attained by means of computed field water-balance patterns which make it possible to determine the watering norms and periods of the irrigation system. This wiI1 provide the possibility of adopting automatic regulation of the watering conditions in a field by taking into consideration the actual moisture requirements of plants. The problem of determining the balance elements and the advisability of such measurement techniques requires for its solution a detailed analysis of the physical prerequisites for the formation of the object's thermal balance. This is related to the fact that the measurements are indirect and, with the exception of certain elements, direct measurement of heat and humidity flows in the bottom layer of the atmosphere is impossible. In order to implement flow measurements it is necessary to have a balance-formation pattern, which should become a basis for the indirect evaluation technique. The bottom turbulent atmospheric layer pattern serves as a physical model for an agricultural field. Its utilization provides the nearest approximation to direct evaluations of the balance elements by measuring the pulsations of temperature, humidity, and the vertical velocity of air flows. The combination of the bottom atmospheric turbulent layer with that of the active soil surface and the vegetable covering makes it possible, by using the energy conservation law when the former is converted in the active layer, to design computed schemes of thermal balance and turbulent diffusion methods. Both these methods postulate the equality of the integral turbulent coefficients of exchange for the transfer of any passive substances (for instance, heat, humidity, etc.) in the turbulent atmosphere. The utilized physical pattern also determines certain basic limitations in the practical application of specific measurement techniques. It is known that the thermal balance is formed under the effect of the received solar radiation and is conditioned by the turbulent transfer of heat and water vapor from the active surface of the field to the atmosphere. Values of the thermal balance elements can be determined for each point in the field, but in this case the appropriate measuring equipment must be located directly on the active surface, whose phygcal representation is to a certain extent indeterminate. According to the accepted concepts regarding bare soil or closely-growing vegetable covering, the active surface level is assumed to be the roughness level determined by the relatively complicated treatment of wind velocity profiles in the bottom atmospheric layer. The active layer becomes even more indeterminate for a loosely-growing vegetable covering, when the nonuniformity of exchange conditions becomes extremely high at adjacent points. Moreover, the application of hypothetical converters directly at the balance formation point will contribute such substantial errors that the actual measurements will become inexpedient. Translated from Izmeritel, naya Tekhnika, No. 8, pp. 25-26, August, 1975.

9 1976 Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $15.00.

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When the measuring devices' sensing elements are placed over the vegetable covering, the mean characteristics of the area are measured, whose size depends on the height at which the transducers are placed above the active surface level and on the turbulent exchange parameters in the bottom atmospheric layer. Representative values of the area's thermal balance elements can be obtained only when the bottom atmospheric layer's turbulent characteristics are relatively stable at the height of the transducers' location. These conditions are interpreted physically as the requirement for the absence of stationary advective (horizontal) heat and humidity flows in the bottom atmospheric layer. Their practical aspect consists of preventing adjacent fields with other characteristics affecting the investigated object. Theoretical and experimental evaluations indicate that, when the transducers are placed at a height of 11.5 m, the measuring point should be removed to 150-200 m from the edge of the field. If the above conditions are met, the calculated values of the heat and humidity flows will represent the energy distribution for the entire field as a whole. This specific feature of measurements can be very valuabIe, since repeated measurements will not be required on the given field and the obtained data can be used in particular for controlling the watering of this field. The initial scheme should determine not only the set of parameters subject to measurement, but also the data processing system which includes the operations of averaging and calculating derived parameters. The existing instruments and devices used at the hydrometerologieal service stations for determining the thermal and water balance elements are not sufficiently precise and the employed technique is based on routine observations by the station personnel five-six times a day and manual processing of the obtained data. Total evaporation is also determined by means of soil evaporators and lysimeters, whose operation is based on approximate patterns of water balance in the upper layers of the soil. These devices are very labor consuming and their error in determining evaporation is large, since investigation is limited to the water balance of an isolated soil section subjected to conditions which are essentially different from the natural ones owing to a disrupted heat and moisture exchange of the soil monolith with the surrounding media. The Agrophysical Scientific-Research Institute has developed a set of instruments and devices for investigating the thermal and water balances in an agricultural field. These installations have different degrees of sophistication and are based on different principles, thus providing the possibility of studying on a wide scale the bottom atmospheric layer and the thermal balance formation on the active surface of the vegetable covering. In addition to the required set of transducers, each installation also comprises a specialized calculator for determining efficiently the thermal and water balance elements directly in the course of measurements. This feature of the equipment is important for its application in remote regions during reconnoitering investigations in the sphere of melioration and the development of new irrigation systems. The heat-balance recorder TB-72M is a device for measuring and calculating heat balance elements (according to the heat-balance method) from the known relationships of the turbulent heat flow to the total field evaporation. The radiation balance and the heat flow into the soil are measured by means of thermoelectric transducers which consist of differential electrolytic copper-constantan thermopiles. The thermopile of the radiation balance transducer is enclosed in averaging reception pads coated with a special actinomettic varnish which is nonselective to radiation wavelengths. The precision of energy balance evaluations by means of the Agrophysical Institute,s equipment is raised by means of special gradient psychrometers which serve to measure directly differences of temperature and humidity without determining their absolute values. Measurements are made by means of differential bridge circuits with sensing elements consisting of thermistors. The methods developed for designing such circuits serve to maintain a constant sensitivity over the working temperature range or to simulate the required relationship between the bridge sensitivity and temperature. The thermal-balance recorder TB-V2M was produced on the basis of an electronic potentiometer type ~PR09 RDM3. In the course of its operation all the measured quantities are converted into voltages and recorded on the potentiometer. One of the spare potentiometer switches is used in the programming unit as a setting device. The recorder's storage components consist of precision potentiometers type PTP-21. Sections of the storage potentiometers (entailing three parameters according to the simulated formula) form during the computation cycles a d c bridge circuit whose fourth arm consists of the [PR output potentiometer's duplicating slide wire. In a balanced bridge the resistance of the duplicating slide wire's working section is proportional to the calculated parameter's value.

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The pulsation method for measuring flows is based on the analysis of the turbulent mechanism entailed in the transfer of passive substances in the atmosphere. For determining the flows it is necessary to register the mutual correlation between the investigated substance's pulsations and the vertical component of the air flow velocity. The Agrophysical Scientific-Research Institute produced a set of instruments for applying the pulsation method in determining the thermal balance elements. It comprises meters of the temperature and humidity pulsations (capacitive hygrometer and semiconductor thermometer), as well as an ukrasonic anemometer. The required information is processed with a specialized correlator. Humidity pulsations are evaluated by using the relationship of air permittivity to humidity. The capacitive humidity transducer controls the measuring generator's frequency. The constant component of humidity as well as other factors which affect permittivity are compensated by means of a double tuned-circuit automatic frequency trimming system which responds to all the slow deviations of the measuring generator's frequency. After intermediate processing the useful component of the signal is fed to the beat detector, The developed pulsating hydrometer model can measure humidity pulsations in the range of 0-3 g / m occurring at 0.01-40 Hz, The pulsation thermometer has a microthermistor with a time constant of the order of 10 -2 sec. ks circuit amplifies the signal and provides a partial compensation of the transducer's inertia. The ulatrasonic anemometer is used for measuring the vertical pulsations of the air flow velocity, The instrument uses a phase method of measurements with correction of errors produced by temperature and the vertical air flow constant component, The developed anemometer model has a base of 50 m m and a velocity measurement range of 0-2 m / s e c in the pulsation range of 0.01-50 Hz. The evaporation and the turbulent thermal flow are evaluated according to the known relationships which are simulated in the course of signal processing by means of the information processing device incorporated in the pulsation equipment. Laboratory and field testing of the equipment developed by the Agrophysical Scientific-Research Institute for determining the heat and water balance elements provided satisfactory agreement with respect to both evaluations and confirmed the improved operational and precision characteristics of the equipment.

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