There are several sensors available that can collect long-term, in situ water potential data over at least a portion of the plant available water range. Our MPS-2 will measure between -5 and -500 kPa, while tensiometers generally measure -85 kPa and wetter. You might also look into thermal resistivity water potential probes or thermocouple psychrometers.

Soil moisture characteristic curves are in common practice to get reliable water potential data for a site. There are not many (if any) in situ instruments available that can capture the entire plant water region of water potential accurately, so soil moisture characteristic curves are a practical and relatively accurate way to get water potential data over a broad range of soil water conditions.

The data you collect with such curves is only as good as the model that you construct and how well it fits your soils. Many soils (particularly clays) show histeresis depending on whether they are wetting or drying.

It's very difficult to collect this type of data without disturbance to the soil. Some sensors have very little impact on the surrounding soils and can be installed down auger holes. You can minimize the disturbance but not entirely eliminate it.