This is one of those uniquely US/Mexico quandaries that shows how intermingled the countries are. As several posters have mentioned, a lot of that agriculture on the Mexican side is tied to the US and either outright controlled by US interests/owners or controlled via purchase arrangements. The for export Ag in Mexico is huge since the NAFTA, during produce season in Pharr alone about 3000 trucks a day cross with just produce and it is expected to increase. That doesn't count Laredo, Browntown, RGC & Roma, or Nogales where a ton crosses also.
Mexico offered to pay the water debt via the dams, that were built after the treaty and do not affect the Falcon or Amistad watersheds, that inflow to the Rio Grande in Rio Grande City. But I don't think the US is accepting the offer as it isn't contemplated in the treaty.
https://www.borderreport.com/news/environment/mexico-proposes-unique-way-to-repay-water-it-owes-us/I don't think a 70 year old treaty could ever have imagined that the Rio Grande Valley and its corresponding Mexican border cities would have a population approaching 3 million people, almost twice the size of San Antonio. The RGV and the Mexican side have a huge water problem, it just hasn't been publicized as of yet as the areas encourage growth as fast as they can. This problem will dwarf Corpus's issues when it comes up. The RGV still has a lot of thirsty Agriculture, as does the Mexican side. Mexico just passed legislation basically nationalizing the countries water rights and access. Mexican Ag groups have marched on Congress, blocked highways, and raised hell, and are continuing to do so. Ain't nobody happy.
The bottom line in all of this is that there is a now ton of farming and ranching in what amounts to semi-desert areas and a boat load of toilets flush and lawns get watered in the lower Rio Grande Delta area, not to mention Laredo. Couple this with HEB, Kroger, and Wal-Mart (and their customers mind you) wanting cheap lettuce, tomatoes, and vegetables year round.
And the 5% tariff will hurt US companies and consumers more than it will Mexico.