40 DIVISION OF WATER RESOURCES 



OPERATION OF TANKS 

 Sources of Errors in Tank Experiments 



In conducting investigations of the consumptive use of water by- 

 plant life two methods, each of which has certain advantages when 

 used under the proper conditions, are available to the investigator. 

 These are as follows : 



1. Determination of consumptive use by field-grown crops by- 



studying soil samples taken with a soil tube. 



2. Quantitative measurements of consumptive use by crops grown 



in tanks. 



Both methods have been used since studies were first begun in this 

 field, but as knowledge of the subject increased there have been improve- 

 ments in both method and equipment. The practice of taking field 

 soil samples at various depths by means of the soil tube for determina- 

 tion of soil moisture is applicable to practically all conditions of soil 

 and crop, and consumptive use of water data may be derived from 

 results so obtained. Similar results may be obtained by measurement of 

 water applied to crops grown in tanks. On account of limitations in 

 the size of tanks that may be used in experimental work, such experi- 

 ments generally include only field or other crops having limited root 

 systems. 



Previous investigators have approved the tank method of studying 

 consumptive use of Avater, though it is subject to some errors which 

 must be overcome as far as possible to secure applicable results. 

 Hence, it is necessary to have some knowledge of the various sources 

 of error. The factors influencing transpiration determinations with 

 rooted plants in tanks as sources of experimental errors have been 

 outlined by Kiesselbach* as follows : 



1. Character of potometer and contents. 



a. Limitation of amount of soil. 



1. Througli size of potometer. 



2. Through number of plants grown in potometer. 



b. Limitation of fertility of soil. 



c. Improper distribution of soil moisture. 



d. Evaporation from surface of soil. 



e. Entrance of rain water. 



f. Exposure of potometer and consequent effect on soil temperature. 



g. Unintentional lack of uniformity in soil. 



2. Environment. 



a. Testing under unnatural habitat. 



3. The plant. 



a. Plant individuality. 



1. Insuflicient number of replications. 



2. Disease and injury. 



b. Stage of maturity. 



1. Insufficient development. 



4. Errors due to methods of computation. 



5. The personal element in drawing conclusions. 



These factors apply also to transpiration plus soil evaporation, 

 which is termed consumptive use of water or sometimes use of water. 

 In the investigation reported herein a number of these sources of error 

 were anticipated and efforts were made to minimize them b}' the selec- 

 tion of tanks of sufficient size to provide soil capacity for proper growth 

 of the types of vegetation chosen for study, by the method of filling 

 the tanks with undisturbed soil and by protection from exposure to 



* Transpiration as a Factor in Crop I'roduction, Nebraska Agricultural Experi- 

 ment Station Research Bulletin No. U, by T. A. Kiesselbach. 1916, 



