Murder on the Mesa — Brett Halliday

In a dry patch of West Texas, Twister and Chuckaluck go to war for water

At first, Twister Malone and Chuckaluck Thompson think it must be a mirage. If real, the willow trees they see on the horizon mean that water is nearby, and in this dusty stretch of the Southwest, water is rarer than gold. They rub the grit from their eyes, yet the sight remains. There really is water up ahead -- and death just around the corner.
 
As the two friends ride toward the trees, they hear the sound of rushing water. Someone has opened a sluicegate, allowing all the precious liquid to drain into the dirt. When Twister and Chuckaluck close the gate and the roaring stops, a woman screams, “Don't let him get away! He was going to kill me!” Chuckaluck fires wildly, but the scoundrel has escaped, setting the wandering cowpokes on the trail of a mystery so dark and twisted it might be beyond their powers to solve.
 
Murder on the Mesa is the 4th book in the Twister and Chuckaluck Mysteries, but you may enjoy reading the series in any order.