Evapotranspiration is when a plant evaporates water off the surface of its leaves in order to move water throughout its system with cohesive and adheisiveforces. My question is how will a sudden decrease in evaportranspiration in regions concentrated with agriculture affect the prevalence of rain in the region? For example, if crops died off in the agricultural region or if the water released via evapotranspiration was collecting in a greenhouse.
Without water in the air, clouds cannot form and evaporate quickly if they do and so a sudden drop in evapotranspiration that resulted in low humidity could decrease the prevalence of rain. But, in simulations, there's not usually much of a measurable effect (http://www.sciencedirect.com/science/article/pii/0273117787902900?np=y). This is because there's a lot more going into this than just evaporation from plants and the total amount of water that winds up in the region's atmosphere.
Air is constantly moving and there's a lot of it, and so to have a measurable impact, this would need to be a pretty large region and/or evapotranspiration would need to stop for an extended period of time.
Plants have a greater impact on their environment than what can be measured through evapotranspiration
Evapotranspiration affects energy balance, which means that sunlight also plays a role in the ultimate effects.
The result is a rather complex web of interactions and an effect that would be very different depending on season, region, and how the loss of evapotranspiration occurred. So, the two scenarios you proposed would have very different effects.