The management methods and parameters were based on published reports and local expertise. ξ Accumulation of heat units to maturity for cotton was estimated using the SWAT-PHU program (https://swat.tamu.edu/ software/; accessed on 24 December 2021).

**Figure 2.** Flowchart of the modeled cultivation practices in the Texas Panhandle.


**Table 3.** Descriptions of simulated scenarios in the Double Mountain Fork Brazos basin.

#### **3. Results and Discussion**

*3.1. Simulated Yearly and Monthly Water Balances in Dry, Normal, and Wet Years under the Alternative Irrigation Application Depths*

In the dry years, the seasonal cotton irrigation amount was 3.5% larger with a small irrigation application depth of 12.7 mm as compared to the baseline irrigation depth of 25.4 mm. In contrast, a 1.8% reduction in seasonal cotton irrigation amount was simulated for the large irrigation depth of 31.8 mm relative to the baseline depth (Table 4). Those changes are a 3.6% increase and a 2.4% decrease for small and large irrigation application depths, respectively, in the normal years (Table 4). However, less than 1% variation was found using the alternative irrigation application depths during the wet years (Table 4), when precipitation was relatively abundant.

An increase in seasonal irrigation amounts with the small irrigation depth led to the increased ETc of 2.1%, 1.5%, and 0.2% in the dry, normal, and wet years, respectively, compared to their respective baseline scenarios. In contrast, ETc decreased by 1.1%, 1.1%, and 0.1% in dry, normal, and wet years, respectively, when using the large irrigation application depth (Table 4). Increasing the irrigation application depth could reduce irrigation frequency to supplement the seasonal water requirements of crops and reduce evaporative water losses associated with irrigation events [34,35]. Evaporative losses associated with irrigation events are greatest during crop vegetative growth periods when incomplete canopy conditions exist. These losses are largely mitigated in quickly maturing crops such as corn (*Zea mays* L.) [8]. However, cotton typically matures at a much slower rate than other agricultural crops, extending the time that the soil surface is subject to evaporative losses following irrigation. Furthermore, cotton may not reach full canopy closure in certain years. As such, less frequent irrigation of greater depth are more likely to result in reduced seasonal irrigation requirements for crops such as cotton.


**Table 4.** Comparison of the average annual water balance parameters and cotton yield under three hydroclimatic regimes using different irrigation application depths, planting dates, and maturity cultivars in the irrigated cotton HRUs in the Double Mountain Fork Brazos basin.

**#** The number in the parentheses is the percent change using an alternative scenario relative to the respective baseline scenario.

Reductions in soil water content, surface runoff, and water yield (the total amount of water leaving the field) were found for the irrigated cotton scenario with the small irrigation application depth under different hydroclimatic regimes compared to the baseline scenarios. However, opposite results were found for the large irrigation application depth scenario under various hydroclimatic years. For instance, soil water content, surface runoff, and water yield decreased by 4.7%, 57.5%, and 2.7%, respectively, with the small irrigation depth, while those hydrologic parameters increased by 2.5%, 115.4%, and 3.3%, respectively, with the large irrigation depth as compared to the baseline scenario in the dry years (Table 4). It is evident that the smaller irrigation depth can result in relatively lower soil water content and runoff. Under the alternative full irrigation management conditions, the cotton yield only showed slight changes (Table 4). There was an increasing trend for cotton yield under the large irrigation depth while a decreasing tendency under the small irrigation depth in diverse hydroclimatic years. Therefore, maintaining/enhancing cotton yield while reducing groundwater pumping from the Ogallala Aquifer in the Texas Panhandle could be achieved using a large irrigation application depth.

The monthly balance analysis showed that the peak irrigation and ETc occurred in July during the dry years (Figure 3a,b) and in August during the wet years (Figure 3d,e) in the irrigated cotton land use. In the dry years of irrigated cotton, there was a high soil water content during the cotton growing season from May to August (Figure 3c). Nevertheless, the soil water content was relatively low in the growing season, especially from July to October during the wet years (Figure 3f). Generally, the smaller irrigation application depth resulted in greater irrigation and ETc (Figure 3a,b,d,e). For example, the irrigation amounts increased by 5.6% and 7.2% in June and July, respectively, in the dry years using the small irrigation application depth compared to the baseline irrigation depth (Figure S1a). The irrigation amount increased by 10.2% in May in the wet years (Figure S1d). The range of increased ETc from May to August varied by 3.0–5.2% and 1.0–1.8% during the dry and wet years, respectively, using the small irrigation depth (Figure S1b,e). The monthly soil water content consistently decreased under the small irrigation depth relative to the baseline irrigation depth in the dry and wet years (Figure 3c and Figure S1c). By contrast, the larger irrigation depth maintained a higher soil water content compared to the baseline irrigation depth (Figure 3f and Figure S1f). In the normal years, overall, the small irrigation depth also led to an increase in irrigation and ETc while soil water content decreased (Figure S2). However, the large irrigation depth caused reductions in irrigation and ETc and maintained a high soil water content.
