Hydrological Validation


The purpose of the Hydrological validation program is to independently assess the benefit of the novel H SAF satellite-derived data on practical hydrological applications. The H SAF concept was born of the requirements of the hydrologic community. Its goal is the development and operational delivery of satellite products that can be used in works pursued by the National Hydrological Services and water management institutions. The H SAF Hydrological Program is focused on the assessment of H SAF products via the so-called hydro-validation (evaluation of products from a hydrological point of view).

The purpose of the Hydrological Validation Program is to assess the benefits of the novel H SAF satellite-derived data on practical hydrological applications and to improve products and their usability in operational hydrology, through the following activities:

- Quality assessment of products and their continuous monitoring by product validation, evaluation and interfacing with hydrological models, performed through impact studies;

- Development: assimilation of satellite data in hydrological models, geospatial analysis, generation of merged (combined) satellite – in-situ products incorporating available ground information to meet expectations of the hydrological community (e.g. requirements on accuracy and resolution), as well as tests of combined products;

- Study of novel satellite products in order to extend, discover or reach an understanding of their applications in operational hydrology,

- Incorporation of emerging user requirements into H SAF periodic assessment,

- Promotion activities (training activities, stimulating the use of satellite products in hydrology and water management, organization and participation in workshops, conferences, webinars).

The main tasks. The tasks carried out by the Hydrological Program include two related, complementary and intertwined issues:

1. Impact Studies and Hydrological Validation of H SAF Products (approaches used to validate products and assess their performance form a hydrological point of view) – work package 5200.

2. HSAF Product Interfacing and utilization improvement (techniques used to improve satellite products and make them more useful for hydrological applications) – work package 5100.


Test sites for hydrological validation are located in: Belgium, Bulgaria, Finland, Germany, Italy, Poland, Slovakia, Turkey. Hydrologic validation is performed by national meteorological and hydrological services, civil protection agencies, research and academic foundations and organizations. The Hydrological models and test sites used are summarized in the following table.

Country (Institution)

Hydrological model

Belgium (RMI)

SCHEME (SCHEldt and MEuse) Data-Based Mechanistic approach

Bulgaria (NIMH)

TOPKAPI, Artificial Neural Networks (ANN), Isba-Modcou model, Mike-11/NAM (Nedbør-Afstrømmings Model)

Finland (FMI)

HOPS (FMI in-house developed model) version 1.4

Germany (Bfg)

HBV (Hydrologiska Byrans Vattenbalansavdelning model)

LARSIM (Large Area Simulation Model

Italy (CIMA)


Poland (IMGW)

HBV (Hydrologiska Byrans Vattenbalansavdelning model)

SRM (Snowmelt Runoff Model)

Slovakia (SMHU)

Hron-NAM (Hron and Nedbør-Afstrømmings Model), HBV (Hydrologiska Byrans Vattenbalansavdelning model)

Turkey (ITU) Turkey (ETU)

Artificial Neural Networks (ANN)


SRM (Snowmelt Runoff Model), HBV (Hydrologiska Byrans Vattenbalansavdelning model)


Validation works are carried out at several related levels. They include:

- standard validation of products;

- assimilation of products to rainfall-runoff models;

- analysis of the improvement of the usefulness of products;

- examples of applications of products;

- case studies.

- In addition to validation, the role of Hydrological Program is to show/present:

- how HSAF products can be used and for what kind of purposes;

- how the usage of HSAF products can be improved;

- how HSAF products can supplement the information from standard measurements and observation network;

- case studies.

The hydrological validation of H SAF products includes:

- calibration of the rainfall-runoff models (using ground data and watershed conditions and or H SAF product data),

- model simulation using ground data (or the equivalent of validated product) as an input (in the validation period),

- model simulation using satellite product as an input (in the validation period),

- comparison of the obtained runoffs with the “measurements” (“observed runoff”).

It should be noted that H SAF products hydrovalidation is based on comparisons between runoff simulated from product data and observed catchment runoff, which is considered as the “truth” in this kind of analysis.

The results of the validation are presented using graphs of observed and simulated runoff (hydrographs – the most convenient method for quick visual inspection of the results) and selected statistical scores – performance measures. The following objectives are considered in product validation:

- the agreement between simulated and observed runoff,

- the agreement between the shapes of simulated and observed runoff hydrographs (including timing, rate and volume of peak flows).

It should be emphasized that a process of evaluation of H SAF products is made primarily by using hydrological models. In contrast to product validation, in which the assessment of product performance is based on a direct comparison between the product and ground-based reference measurements, hydrovalidation is based on “indirect comparison”.

Product validation and hydrologic validation philosophy (on an example of precipitation products)

Product validation is a comparison between product values and its reference, ground data; this is a direct comparison between characteristics of the same type (e.g. estimators of precipitation amounts from HSAF products are compared with precipitation estimates from rain gauges or radar rainfall products). On the other hand, hydrologic validation means assessment of HSAF products according to their impact on runoff simulation at the outlet of the test basin; it is based on an indirect comparison between discharges simulated using the product as model input and discharges observed at water gauge station.

The most frequently used hydrological models accept only specific input data. From precipitation, snow cover and soil moisture products, which are the subject of hydrological validation, only precipitation products are accepted straightforwardly as a time series. Snow cover and soil moisture products need to be assimilated or integrated into models first.