There's a detailed description of perhaps the most comprehensive and sympathetic effort to test dowsing ability at this link:
https://skepticalinquirer.org/1999/01/testing_dowsing_the_failure_of_the_munich_experiments/
The German government spent a quarter million marks in 1980s money. The results were indistinguishable from random chance, and were in fact worse than if the dowsers had left their rods at home and always guessed for
water at the middle of the test area.
Everybody has an uncle whose well driller found water where nobody else could this way. You don't hear so much about the ten thousand dry
wells from when it did not work. I remain a believer in the scientific method. Non-reproducible methods that lack an explainable mechanism of action do not impress me.
Readers may also be interested in this detailed 1917 publication of the United States Geological Survey:
The Divining Rod: A History of Water Witching, with a Bibliography
The use of a forked twig, or so-called divining rod, in locating minerals, finding hidden treasure, or detecting criminals is a curious superstition that has been a subject of discussion since the middle of the sixteenth century and still has a strong hold on the popular mind, even in this country, as is shown by the large numiber of inquiries received each year by the United States Geological Survey as to its efficacy, especially for locating underground water, and the persistent demands that it be made a subject of investigation by the Survey. The bibliography shows that a truly astonishing number of books and pamphlets have been written on the subject. The purpose of the present brief paper is not to add another contribution to this enormous volume of uncanny literature but merely to furnish a reply to the numerous inquiries that are continually being received from all parts of the country. The outline of the history of the subject presented in the following pages will probably enable most honest inquirers to appreciate the practical uselessness of "water witching'' and other applications of the divining rod, but those who wish to delve further into the mysteries of the subject are referred to the literature cited in the bibliography, in which they will find reports in painful detail of exhaustive investigations and pseudo-investigations of every phase of the subject and every imaginable explanation of the supposed phenomena.
It is doubtful whether so much investigation and discussion have been bestowed on any other subject with such absolute lack of positive results. It is difficult to see how for practical purposes the entire matter could be more thoroughly discredited, and it should be obvious to everyone that further tests by the United States Geological Survey of this so-called ''witching'' for water, oil, or other minerals would be a misuse of public funds.
...
To all inquirers the United States Geological Survey therefore gives the advice not to expend any money for the services of any "water witch"...