Check into Tensiometers or other sensors. Here is one company.

irrometer.com

Tensiometers measure the water tension usually in centibars (KPa) with a Gauge.

Video of Tensionometer Installation

https://www.youtube.com/watch?v=UHVlvAO5NDQ

https://www.youtube.com/watch?v=f66mQeHYxCE

You can also Evapotranspiration Rates to schedule irrigation.
Monitoring the evaporation water loss from a pan of water. This is usually in mm
or inches per day. You should know the usable water in your soil type for the
depth desired in inches or mm of water. One inch of water is about 0.623 Gallons
per sqft. You can irrigate at the 50% point or Replace lost water at more
frequent intervals. Below are links with info. Average Evaporation rates in the
US per each month of the year. Plus misc info.

http://www.etgage.com/04707.html

http://www.waterandclimatechange.eu/evaporation/average-monthly-1985-1999

https://en.wikipedia.org/wiki/Evapotranspiration

https://en.wikipedia.org/wiki/Pan_evaporation

This method works good for daily drip irrigation applications, Using the water use per day. The water deficient method of Irrigation. Evaporation monitors can made for Plastic Basins( Earth Colors best). Just cover with mesh to kept animals from drinking etc.