Saudi rainfall does not behave like the rainfall most drainage software was written for. Annual totals are tiny — 50–150 mm across much of the Kingdom — but they arrive in a handful of short, violent convective events. A wadi that has been dry for three years can carry a destructive flood in under an hour. Design criteria built on temperate-zone assumptions fail here in both directions: they over-size networks for storms that never come, and under-size them for the one that does.
This note describes the approach we use to build rainfall inputs for stormwater modelling on KSA projects — one that has survived municipal review under MOMRAH requirements and tier-one consultancy QA.
The data problem comes first
The gauge network is sparse and the records are short. Outside the major cities, you will commonly find daily-read gauges with 25–40 years of record, very few sub-daily recording gauges, and significant gaps. Three consequences follow:
- Annual-maximum series are short, so the fitted distribution's upper tail — exactly the part you design against — carries wide confidence bounds.
- Sub-daily ratios must usually be inferred, not measured, because the gauges record daily totals only.
- Spatial interpolation is unreliable in convective climates: a storm that floods one catchment can miss the gauge 15 km away entirely.
We supplement gauge records with satellite rainfall products (CHIRPS and similar) for spatial context and record extension — not as a replacement for gauges, but to test whether a fitted distribution is plausible against a longer regional signal.
Frequency analysis that survives review
For the at-site analysis we fit candidate distributions — Gumbel, GEV, Log-Pearson III — using L-moments rather than ordinary moments. L-moments are markedly more robust on the short, skewed records typical of arid regions, and the choice is easy to defend in a design report. Where the at-site record is under ~30 years, we use a regional frequency approach: pool standardized records from hydrologically similar stations, fit a regional growth curve, and rescale by the at-site mean.
Practice note: when two distributions fit the record almost equally well but diverge at the 100-year quantile, do not silently pick one. Carry both through to flows, show the sensitivity, and let the criticality of the asset decide. Reviewers respect a stated uncertainty far more than false precision.
From daily depths to IDF curves
With design daily depths established per return period, sub-daily IDF construction follows from ratio methods — typically Bell-type or regionally calibrated ratios where municipal IDF tables don't already exist. Two regional cautions:
- Short-duration intensities are ferocious. Convective cells routinely deliver a large fraction of the daily total inside 30–60 minutes. If your 1-hour/24-hour ratio looks like a UK or German value, it is wrong for KSA.
- Check whether the municipality mandates its own IDF. Riyadh, Jeddah and several Royal Commission jurisdictions publish criteria; MOMRAH guidance applies broadly. Where a mandated curve exists, your derived curve serves as a sanity check, not a substitute.
Areal reduction, hyetographs and climate scaling
Point rainfall over-estimates catchment-average rainfall, and the bias grows with catchment size — sharply so in convective climates. For catchments beyond a few square kilometres we apply areal reduction factors appropriate for arid convective storms, which decay faster with area than the temperate ARFs embedded in many manuals.
For design hyetographs we favour front-loaded or Chicago-type profiles at short durations, reflecting observed storm structure. For 2D rain-on-grid models in HEC-RAS or InfoWorks ICM, the hyetograph shape materially changes peak flow and flood extents — it is a design decision, not a software default.
On climate change: IPCC AR6 projections for the Arabian Peninsula indicate intensification of extreme short-duration rainfall even where annual totals fall. We apply an uplift to design intensities on critical infrastructure — stated explicitly, agreed with the client, and recorded in the basis-of-design.
What goes in the basis-of-design report
- Gauge inventory, record lengths, gap treatment and quality screening
- Distribution selection with L-moment ratio diagrams and goodness-of-fit evidence
- Derived IDF table per return period and duration, against any mandated municipal curve
- ARF and hyetograph selection with justification
- Climate uplift, stated and signed off
- Code trail: MOMRAH and project-specific criteria, with international references (TR-55, USACE EM, FHWA) where they fill gaps
That single document is the difference between a smooth municipal approval and a three-cycle review. It is also, candidly, the deliverable most often missing when we are brought in to rescue a stalled drainage package.
Where this feeds next
The IDF curve is an input, not an answer. It drives the rainfall-runoff model, the rain-on-grid 2D simulation, and ultimately the network sizing and flood mitigation design issued to IFC. We cover the modelling-tool decision in a companion note: HEC-RAS 2D vs InfoWorks ICM — an engineer's honest comparison.
Working on a KSA drainage package and want the hydrology done defensibly? Send us the brief — a principal replies within two business days.