4.2 Hydrologic Soil Groups, Disturbed Profiles, and Time Parameters

Soil Permeability, Infiltration Capacity, and Antecedent Moisture

Whether rainfall soaks into the ground or becomes erosive overland flow depends on the physical properties of the soil profile. Infiltration capacity ($f$) is the maximum rate (in inches per hour, $\text{in/hr}$) at which a soil surface can absorb falling precipitation at a given moment. Once water crosses the surface, its downward percolation is governed by soil permeability (quantified as saturated hydraulic conductivity, $K_{\text{sat}}$), which is determined by the size, continuity, and connectedness of soil pores rather than total pore volume alone.

Five Factors Governing Soil Infiltration Rate

1. Pore Size Distribution and Soil Texture: Coarse-textured soils (sands and gravels) contain large macropores that drain rapidly under gravity, yielding high infiltration and hydraulic conductivity. Conversely, fine-textured soils (clays and silty clays) often have higher total porosity than sand, but their pores are microscopic micropores that hold water tightly by capillary tension and transmit water extremely slowly.
2. Antecedent Moisture Condition (AMC) / Antecedent Runoff Condition (ARC): When a storm begins on dry soil, strong capillary suction pulls water rapidly into empty pores, producing a high initial infiltration rate ($f_0$) that gradually decays toward a steady-state minimum saturated rate ($f_c$) as pores fill. The USDA Natural Resources Conservation Service (NRCS) categorizes pre-storm moisture into three Antecedent Runoff Conditions:
   • ARC I (AMC I — Dry): Soils are dry (low antecedent rainfall over the prior $5\text{ days}$); initial infiltration capacity is high, effective Curve Numbers are reduced, and runoff volume is low.
   • ARC II (AMC II — Average / Normal): The standard baseline moisture condition used in published NRCS TR-55 Curve Number tables and standard engineering design.
   • ARC III (AMC III — Wet / Near-Saturated): Heavy rainfall or low evapotranspiration over the preceding $5\text{ days}$ has filled soil pore spaces. Infiltration capacity starts near its minimum saturated rate ($f_c$), effective Curve Numbers increase substantially, and even moderate storms generate rapid ponding and large runoff volumes.
3. Vegetative Cover and Organic Matter: Living plant roots, decaying root channels, earthworm burrows, and soil organic matter bind mineral grains into stable aggregates (peds) that maintain continuous macropores.
4. Surface Crusting and Sealing: When bare, unmulched soil is exposed to raindrop impact, kinetic energy shatters surface soil aggregates (slaking). Detached fine silt and clay particles wash into the upper $1\text{ to }3\text{ mm}$ of soil pores, forming a dense, nearly impermeable surface seal or crust. A thin surface crust can slash infiltration rates by $70\%\text{ to }90\%$ even when the underlying subsoil remains completely dry.
5. Mechanical Compaction: Construction traffic—scrapers, haul trucks, dozers, and sheep's-foot rollers—compresses soil particles into a tight packing arrangement, increasing soil bulk density ($\rho_b$) and crushing transmission macropores.

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NRCS Hydrologic Soil Groups (HSG A, B, C, and D)

The NRCS groups soils by runoff potential when thoroughly wet, using saturated hydraulic conductivity in the relevant soil layers, depth to a restrictive layer, and depth to the high water table. Texture is useful evidence but is not a complete classification method. A field surface-infiltration measurement is not interchangeable with the profile criteria.

The June 2025 NRCS HSG criteria, checked October 13, 2026, use different conductivity cases according to profile depth. For one specific case—water table deeper than 100 cm and no applicable shallow restriction—the least transmissive layer within the upper 100 cm is classified using conductivity greater than 1.42 in/h for A, greater than 0.57 through 1.42 in/h for B, greater than 0.06 through 0.57 in/h for C, and no more than 0.06 in/h for D. Do not apply that case to every shallow or wet soil. For shallower profiles, use the complete table and its depth criteria. Dual groups apply to wet soils that can be drained, and the actual drainage condition must support use of the drained letter.

A mapped group and a design curve number answer different questions. Curve number also depends on cover, treatment, hydrologic condition, and the runoff assumptions. Bare-earth values such as 77, 86, 91, and 94 are the particular TR-55 newly graded-area values used in this guide’s example, rather than universal values for every site with the same HSG. Verify the land cover and condition in the approved runoff calculations.

Impact of Disturbed Construction Profiles on HSG Behavior

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Watershed Time Parameters ($T_c$, $T_t$, and $T_L$) and Flow Regimes

To determine how fast runoff concentrates at a discharge point—and therefore how intense the design storm is on an IDF curve—hydrologists evaluate three interrelated time parameters:

1. Time of Concentration ($T_c$)

Time of Concentration ($T_c$) is defined as the time required for surface runoff to travel from the hydraulically most distant point in the watershed to the watershed outlet or design point of interest.

• Hydraulically Most Distant vs. Geographically Most Distant: On the CESSWI exam, remember that the hydraulically most distant point is the point with the longest travel time, which is not necessarily the point farthest away in straight-line feet. For example, a $400\text{-foot}$ flow path across a flat, dense turf buffer strip ($V = 0.25\text{ ft/s}$, taking $1,600\text{ seconds}$) is hydraulically more distant than a $900\text{-foot}$ flow path down a steep, paved haul road ($V = 4.5\text{ ft/s}$, taking $200\text{ seconds}$).
• Why $T_c$ Governs Peak Discharge: A storm must last at least as long as $T_c$ for water falling at the upper hydraulic boundary to reach the outlet at the same time as water falling near the outlet. Thus, full-basin contribution occurs when storm duration equals $T_c$.

2. Segment Travel Time ($T_t$)

As runoff moves from the watershed divide to the outlet, it passes through sequential hydraulic segments. The Travel Time ($T_t$) through any individual segment of length $L$ ($\text{ft}$) at average velocity $V$ ($\text{ft/s}$) is:

$$T_t = \frac{L}{3600 \times V}$$

where $T_t$ is expressed in hours ($\text{hr}$) and $3600$ converts seconds to hours. The total Time of Concentration ($T_c$) is the sum of the segment travel times along the critical hydraulic path:

$$T_c = T_{t1} + T_{t2} + \dots + T_{tm} = \sum_{i=1}^{m} T_{ti}$$

3. Watershed Lag Time ($T_L$)

Lag Time ($T_L$) is the time interval measured on a storm hydrograph from the center of mass (centroid) of excess rainfall to the peak rate of runoff ($Q_p$). In NRCS unit hydrograph methodology (TR-55), lag time is approximated as $60\%$ of the time of concentration:

$$T_L = 0.6 \times T_c$$

The Three Sequential Hydraulic Flow Regimes in NRCS TR-55

When calculating $T_c$ using NRCS Technical Release 55 (TR-55), overland flow is divided into three sequential hydraulic flow regimes from the ridge divide to the outlet:

1. Sheet Flow: Very shallow, uniform flow (typically $< 0.1\text{ ft}$ deep) moving over plane surfaces at the upper headwaters of a sub-basin. The original 1986 TR-55 allowed sheet-flow lengths up to 300 feet; later NRCS guidance commonly uses 100 feet or less. Apply the method adopted in the approved design and observed surface conditions rather than call 100 feet the original TR-55 limit. Sheet flow travel time is calculated using Kinematic Wave / Manning's kinematic equation:

$$T_t = \frac{0.007 (n L)^{0.8}}{(P_2)^{0.5} S^{0.4}}$$

where $n$ is Manning's sheet-flow roughness coefficient (which reflects raindrop impact and shallow drag and is much larger than open-channel $n$, e.g., $n = 0.011$ for smooth pavement/bare packed soil vs. $n = 0.15\text{ to }0.24$ for dense grass and $n = 0.40\text{ to }0.80$ for woods with underbrush), $L$ is sheet flow length ($\le 100\text{ ft}$), $P_2$ is the $2\text{-year, 24-hour}$ rainfall depth ($\text{in}$), and $S$ is land slope ($\text{ft/ft}$). 2. Shallow Concentrated Flow: Beyond the first $100\text{ feet}$ (or sooner on bare, erodible construction grades), sheet flow converges into micro-topographic swales, rills, wheel ruts, or street gutters at depths of roughly $0.1\text{ to }0.5\text{ ft}$. In TR-55, average velocity ($V$, in $\text{ft/s}$) is determined solely from watercourse slope ($S$, in $\text{ft/ft}$) and surface type:

• Unpaved surface: $V = 16.1345 \times S^{0.5}$
• Paved surface: $V = 20.3282 \times S^{0.5}$

1. Open-Channel or Pipe Flow: Shallow concentrated flow eventually enters a defined roadside ditch, diversion swale, stream channel, or storm drain pipe. Velocity in this regime is calculated using Manning's Open-Channel Flow Equation based on channel cross-sectional geometry, bed slope, and boundary lining roughness.