hello! sort of new to the site myself, so I am really not one that can tell you how to pack lite, but I am from Texas and now a good bit about Big Bend. are you from the area? The reason I ask is that depending on where you go in Big Bend will depend on what you need to take. If you are staying at the lower altitudes, be very careful in that your days can be very warm, while nights will get very cold. The higher altitudes will be a bit more moderate during the day but very cold at night. As with any terrain, this temperature description is the "norm" for that area and a cold front could change how cold your days are. With desert terain you will need to pack for both extremes as you do not know what you are going to get in terms of weather out there. Most terrain will give you a temperature fluctuation of 30 degrees between night and day, however desert is much greater than that. Nights can be 50 degrees cooler sometimes and you need to pack for both. You will need to pack in all your water unless you will be hiking near the river and be sure to take water purification (not just filtration) if you go there...That river is shared with Mexico and while most N America water is pretty virus free, I would not trust that river. They state to watch out for illegals in the area but I have never had any problems with them although I have seen quite a few.
There are a few streams or creeks here and there on the trails but you will need to know that they are on the trails you are taking to be able to count on them being an option for you to suppliment your supply. Also keep in mind you will need to double how much you drink in those conditions, whether it is cold or not. The dry air will take hydration out of you very quickly... it is more dangerous in the cold than in the heat because you dont feel as thirsty in the cold.