TX069-PLAYAS

Overview

The TX069 playas site covers 324.74 km² of the Texas High Plains, a landscape defined by what it lacks: integrated drainage. Runoff collects in thousands of shallow, internally drained playa depressions rather than flowing to streams. With 61.3 m of total relief spread across the largest study area in the ensemble (5.4 million cells at 8 m resolution) and an ARS of 0.07, this is the flattest site by a wide margin, the opposite end of the roughness ranking from Coweeta (5.5).

Playas are a hard case for overland flow models. Flow directions are weak, ponding dominates routing, and small elevation errors can reroute water between adjacent basins. However, they matter: playas are the primary recharge points for the Ogallala Aquifer, so getting their contributing areas right has direct consequences for water management on the High Plains.

What this site demonstrates:

SIMWE behavior in depressional, internally drained terrain where ponding replaces channelized flow.
Scalability: the largest cell count in the study by an order of magnitude.
How watershed delineation degrades with resolution when relief approaches the DEM error budget.

Site Details

Location	Texas High Plains playas (soil survey area TX069), Texas
CRS	WGS 84 / UTM zone 13N (EPSG:32613)
DEM resolution	8 m
NAIP year	2022
Area	324.74 km² (5,378,306 cells)
Elevation	1104.0 - 1165.3 m (mean 1134.9 m)
Average roughness (ARS)	0.07

NAIP Imagery and Derived Products

Geomorphology

1st Order Derivatives

2nd order topographic derivatives

Slope	Aspect	Geomorphon

Profile Curvature	Planform Curvature

Soils

Soil survey polygons come from SSURGO, rasterized by map unit key (MUKEY). The MUKEY raster links every cell to its soil map unit properties (e.g., hydrologic soil group, saturated hydraulic conductivity), which drive the curve-number and infiltration inputs to the simulations below.

SSURGO MUKEY

Simulations

Overland flow is simulated with r.sim.water, the SIMWE path-sampling solution of the shallow-water flow equations. The default run applies a uniform 50 mm/hr rainfall with no infiltration and Manning’s n of 0.1, writing water depth (m) and unit discharge (m³/s) every 2 minutes over a 10-minute event. Spatial patterns therefore reflect terrain routing alone; the ground water and sensitivity sections below relax these assumptions.

Default Parameters

Depth

2 Min	4 Min	6 Min	8 Min	10 min

Discharge

2 Min	4 Min	6 Min	8 Min	10 Min

Ground Water

These runs replace the uniform rainfall input with stream-fed sources derived from flow accumulation, using 500,000 walkers and a 4-minute output step (scripts/groundwater.py). Three scenarios are simulated: groundwater seepage in streams during a 10 mm/hr rainfall, seepage with no rainfall, and springs at first-order streams only. These runs end at 19 minutes. Depth is in meters, discharge in m³/s.

Depth - Groundwater seepage in streams during rainfall

4 Min	8 Min	12 Min	16 Min	19 Min

Discharge - Groundwater seepage in streams during rainfall

4 Min	8 Min	12 Min	16 Min	19 Min

Depth - Groundwater seepage in streams, no rainfall

4 Min	8 Min	12 Min	16 Min	19 Min

Discharge - Groundwater seepage in streams, no rainfall

4 Min	8 Min	12 Min	16 Min	19 Min

Depth - Springs

4 Min	8 Min	12 Min	16 Min	19 Min

Discharge - Springs

4 Min	8 Min	12 Min	16 Min	19 Min

Sensitivity Analysis

The sensitivity analysis re-runs the simulation across DEM resolutions of 1, 3, 10, and 30 m and walker densities of 0.25 to 2 walkers per cell, with 10 random seeds per combination and 120 one-minute iterations (scripts/sensitivity.py). The watershed extent is delineated at each resolution with a 30,000-cell accumulation threshold.

Basin extent by resolution

Overlap of watershed extents delineated at 1, 3, 10, and 30 m resolution

Seed-averaged depth by resolution

Mean water depth across the 10 seeds at the final output step of each run (walker density 2). The coarser grids end a few steps early: 117 at 10 m and 118 at 30 m.

1 m	3 m	10 m	30 m