UNAUTH LNDG ON RAMP INSTEAD OF RWY.

Narrative

I HAD 2 INST STUDENTS ABOARD IN AN SMA. ONE OF THE STUDENTS HAD BEEN PRACTICING VOR TRACKING WITH COURSE REVERSALS FOR ABOUT 30 MINS AT SGD VOR. I BROKE OFF THE EXERCISE TO PROCEED TO SONOMA SKYPARK FOR LNDG WITH INTENTION TO CHANGE STUDENT PLTS. WE HAD NO SOONER LEFT THE VOR WHEN THE STUDENT PAX EXPRESSED A PRESSING NEED FOR THE BATHROOM. AT AGE 69 I AM VERY MUCH AWARE OF HOW PRESSING THIS NEED CAN BECOME. I IMMEDIATELY DIVERTED TO SONOMA VALLEY. IT HAD BEEN RAINING IN OUR AREA THE NIGHT BEFORE AND I WAS UNCERTAIN AS TO THE CONDITION OF THE DIRT/GRAVEL RWY 27/9 SO I OPTED FOR WHAT I KNEW AS RWY 34; A PAVED STRIP. BECAUSE OF MY SENSE OF URGENCY BY WHAT I CONSIDERED A POTENTIAL COCKPIT CATASTROPHE I EXECUTED A PWR OFF SHORT APCH TO WHAT I HAD KNOWN FOR YRS TO BE RWY 34. THE RWY WAS NOT RWY 34 BUT A RAMP I DO NOT KNOW HOW RECENT THE CHANGE BUT BECAUSE OF MY PAX PHYSICAL NEEDS I CHOSE TO LAND. I USED LESS THAN 1/2 OF THE EXISTING RWY; EXECUTED A 180 DEG TURN AND DEPLANED MY PAX. WHEN HE RETURNED TO THE ACFT WE DEPARTED VIA RWY 9 WHICH I DETERMINED TO BE DAMP BUT SUITABLE.