Thank you for all the responses. Lulu has been getting rinsed off after riding. My trainer said at first Lulu hated it but she seems to be tolerating it now. Choosing a warm day isn't a problem. Our spring temperatures are what most people consider extremely hot! And our ground water is not very cold so the water coming out of the hose isn't making her chilly. She doesn't like the water close to her face. Today, her face was dirty so I just wiped it down with a small terrycloth towel. By the way, when I first got her she wouldn't tolerate a cloth coming near her face so every time I groomed her I would have a small towel and would quickly wipe near her face. She is now very good about letting me clean her face. Today she had some goop in her eye and allowed me to take the cloth and gently wipe it over her eye.
I will remember to bring a change of clothing and something other then my riding boots!
The rolling! One of the first times I rinsed Lulu off after a ride I asked the trainer when I could put her back in her stall. She said if I didn't mind Lulu rolling and getting dirty I could put her away wet. If I wanted a cleaner horse I should wait until Lulu was dry. Good call about the sand arena. I'll make sure she is completely dry and then let her out in the round pen or the arena so if she wants to roll it will be in the sand.
On a side note, when we built our house I asked my husband to give me a couple of outdoor faucets with hot water so we have one in the backyard and one in the front. Our winters are not that cold and it was so nice to wash the car with warm water in the bucket (and not have to bring it from inside the house). Also, we used to have a German Shepherd and it was nice to have warm water to bathe him outside in the winter. Another plus was when my daughter was a toddler and wanted to play in her kiddie pool. The outside temperature was warm enough but the water was a bit chilly. My husband used to get so exasperated. "You're putting bath water in her pool!" Um, yes, I just got 45 minutes to read my book in peace while the small person splashed around.