To understand how floods happen, and how early warning systems can anticipate them, you first need to understand the water cycle: the continuous process by which water moves between the atmosphere, the land surface and the ground beneath it. This article explains, accessibly but rigorously, how rain becomes runoff, what determines a catchment’s response and how these concepts are applied in flood forecasting.

The water cycle: an overview

On a planetary scale the water cycle is a closed system. The total amount of water on Earth is practically constant, around 1,386 million cubic kilometres, but it is continuously redistributed between reservoirs: the oceans (96.5 %), ice caps and glaciers (1.74 %), groundwater (1.69 %) and a tiny fraction in rivers, lakes, soil and the atmosphere.

The main processes are:

  • Evaporation and transpiration. Water from oceans, lakes, rivers and soils evaporates under solar energy. Plants also release water vapour through their stomata. Together these processes are called evapotranspiration.
  • Condensation. Vapour rises, cools and forms clouds as it condenses onto condensation nuclei such as dust or sea salt.
  • Precipitation. When droplets or ice crystals grow large enough, they fall under gravity as rain, snow, hail or sleet.
  • Interception. Part of the precipitation is held by vegetation — tree canopies, leaf litter — before it reaches the ground.
  • Infiltration. Water reaching the ground penetrates through the soil pores towards deeper layers.
  • Runoff. Water that neither infiltrates nor is intercepted flows over the surface towards channels, rivers and ultimately the sea.
  • Percolation. Infiltrated water can descend as far as the aquifers, feeding the base flow of rivers.
ⓘ Key figure Of all the rain falling on the Iberian Peninsula — some 346,000 cubic hectometres a year according to Spain’s Water White Paper — roughly two thirds evapotranspires, and only a third becomes surface runoff or groundwater recharge, equivalent to about 111,000 cubic hectometres of renewable water resources per year.

Types of precipitation

Not all precipitation carries the same flood risk. In hydrology it is crucial to distinguish between:

  • Frontal precipitation, associated with warm, cold or occluded fronts. It tends to be widespread and moderate in intensity, and is typical of Atlantic Spain in winter.
  • Orographic precipitation, produced when moist air masses are forced to rise over a mountain barrier, cooling and condensing. Highly relevant in the coastal ranges of the Mediterranean.
  • Convective precipitation, driven by strong updraughts that build cumulonimbus clouds. It produces local storms of great intensity and short duration, and it is the most dangerous for flash flooding.

The critical parameter is not so much the total rainfall as its intensity in millimetres per hour. A hundred millimetres spread over 24 hours rarely causes flooding; the same amount in two hours can produce a catastrophe.

Infiltration versus runoff: the battle in the soil

When rain reaches the ground, two processes compete: infiltration, the water the ground absorbs, and surface runoff, the water that flows over it. The outcome of that competition determines whether a flood occurs.

What controls infiltration

  • Soil type. Sandy soils are highly permeable and can absorb more than 200 mm per hour, while clay soils are far less permeable, under 5 mm per hour once saturated.
  • Antecedent moisture. Soil already wet from previous rain has much less absorption capacity. This is one of the most critical factors: moderate rain on saturated soil generates far more runoff than intense rain on dry soil.
  • Vegetation cover. Vegetation slows surface flow, encourages infiltration through root channels and holds water by interception. A mature forest can intercept between 15 % and 40 % of precipitation.
  • Slope. On steep hillsides water has less time to infiltrate and concentrates quickly in the channels.
  • Land use. Urbanisation, agricultural compaction and deforestation drastically reduce infiltration capacity.

Types of runoff

Hydrologists distinguish several runoff generation mechanisms:

  • Hortonian runoff, or infiltration-excess runoff, occurs when rainfall intensity exceeds the soil’s infiltration capacity. It is the dominant mechanism in arid and semi-arid areas, which covers much of Mediterranean Spain.
  • Saturation-excess runoff occurs when the soil is completely saturated and can absorb no more, regardless of rainfall intensity. It is typical of areas with a high water table and humid climates.
  • Subsurface flow, or interflow, is infiltrated water moving laterally through the soil above an impermeable layer. Slower than surface runoff, it contributes to river discharge with some delay.

Time of concentration

One of the most important concepts in flood hydrology is the time of concentration: how long water takes to travel from the most remote point of a catchment to its outlet. This parameter determines how long after the rain starts the peak discharge will arrive.

It depends on:

  • Catchment size. Large catchments have longer times of concentration, measured in hours or days, allowing more time to react.
  • Average slope. The steeper the catchment, the shorter the time.
  • Length of the main channel. Longer channels mean longer times.
  • Surface roughness. Smooth surfaces such as asphalt or rock speed the flow; rough surfaces such as dense vegetation slow it.
⚠ Dangerous catchments Typical Mediterranean ramblas have times of concentration of just 30 minutes to two hours. That means less than an hour can pass between heavy rain starting and the flood peak arriving. By comparison, large Atlantic basins such as the Ebro or the Duero have times of concentration measured in days.

The SCS-CN method: a simplified runoff estimate

The Curve Number method, developed by the US Soil Conservation Service, is one of the most widely used approaches in the world for estimating direct runoff from rainfall. Its popularity rests on its simplicity: it needs only one parameter, the Curve Number.

The Curve Number is a value between 0 and 100 reflecting a catchment’s capacity to generate runoff:

  • Low (30–50): permeable soils with good vegetation cover and little runoff, such as forest on sandy soil.
  • Medium (50–70): soils of intermediate permeability under agricultural use, with moderate runoff.
  • High (70–90): poorly permeable soils or sparse vegetation, with high runoff, such as clay soil with poor pasture.
  • Very high (90–98): impermeable or near-impermeable surfaces where almost all rain becomes runoff, such as an urban area or an asphalt car park.

The basic formula holds that, once rainfall exceeds an initial abstraction threshold, runoff increases non-linearly, accelerating as the soil saturates. That non-linear behaviour explains why the worst floods occur when intense rain falls on ground that is already wet.

The unit hydrograph: a catchment’s fingerprint

A hydrograph is the graph showing how a river’s discharge varies over time during and after rainfall. It has a characteristic shape: a rapid rise to the peak, followed by a more gradual decline known as the recession curve.

The unit hydrograph concept, introduced by Sherman in 1932, holds that every catchment has a characteristic response to uniform rainfall of unit duration and intensity. Knowing that response, hydrologists can predict the catchment’s behaviour under any rainfall event through a mathematical process called convolution.

The key parameters are:

  • Time to peak: the interval from the start of effective rainfall to maximum discharge.
  • Peak discharge: the maximum flow reached, which determines the potential severity of the flood.
  • Base time: the total duration of the direct runoff hydrograph.
  • Runoff volume: the area under the curve, equivalent to the total volume of water passing the control point.

Urban hydrology: when asphalt multiplies the risk

Urbanisation radically transforms a catchment’s hydrological response. Impermeable surfaces — asphalt, concrete, roofs — virtually eliminate infiltration, turning almost all rainfall into direct runoff.

The effects are dramatic:

  • More runoff volume. A natural catchment may generate 10 to 20 % runoff; the same catchment urbanised can generate 80 to 95 %.
  • Shorter time of concentration. Sewer networks and smooth surfaces accelerate the flow, cutting the time of concentration by 50 to 80 %.
  • Higher peak discharge. As a result of the two factors above, the peak can be two to six times higher than in natural conditions.
  • Less aquifer recharge. Less infiltration means less groundwater and lower river base flow in the dry season.
ⓘ Sustainable urban drainage To counter these effects, more and more cities are adopting sustainable drainage systems: permeable paving, rain gardens, green roofs and retention ponds that mimic natural hydrological behaviour. Cities such as Vitoria-Gasteiz, Barcelona and Madrid already build these solutions into their planning.

From theory to forecast: how a flood is modelled

Modern flood forecasting systems, including those feeding WhatAWeather, combine all of these concepts into mathematical models that work in three steps:

  • Estimating the rainfall. Using weather radar, rain gauges from the SAIH network and numerical forecast models, the system determines how much rain will fall, where and for how long.
  • Converting rainfall to runoff. Using methods such as SCS-CN, and taking antecedent soil moisture into account, it calculates what fraction of that rain will become direct runoff.
  • Routing the hydrograph. Using the unit hydrograph or more sophisticated methods such as Muskingum-Cunge, it predicts how that runoff will translate into discharge at each point of the river network, and when it will reach each town.

The quality of these forecasts depends critically on the spatial and temporal resolution of the input data and on properly calibrating the model parameters for each specific catchment.

How this connects to WhatAWeather

WhatAWeather combines Open-Meteo weather forecasts with SAIH network information — real-time river and reservoir levels — to provide early flood warnings. The system takes into account:

  • Rainfall forecasts for the next 48 hours, including intensity and spatial distribution.
  • Current river levels at SAIH gauging stations.
  • The state of reservoirs: one at 90 % of capacity has far less attenuation margin than one at 50 %.
  • Open-Meteo flood models, which capture the hydrological response of the main European basins.

Understanding the concepts in this article — why soil type matters, what time of concentration means, how urbanisation multiplies risk — lets you read warnings better and make more informed decisions when risk arrives. Hydrology is not magic: it is the physics of water, and the better we understand it, the better prepared we are.

Water always obeys gravity. Knowing the catchment you live in — its geology, its vegetation, its slopes — is the foundation of any preparation against flooding.