Precipitation

Before you can study any hydrologic process occurring within a watershed, you need to have some idea of when and where the watershed is likely to receive precipitation. Let's start with an introduction of the topics covered in this module to summarize understanding of how precipitation is distributed in space and time (3:42 min).

Precipitation comes from water vapor in the air. Let's start with a review of how we characterize the amount of water in the air (8:08 min).

Understanding precipitation of water from the air first requires understanding the temperature to which the air must be cooled for water vapor to be at saturation (2:15 min).

With an understanding of vapor pressure and the dew point, we can review the conditions necessary to produce a meaningful amount of precipitation from the atmosphere (5:18 min).

Global patterns in precipitation are a primary predictor of a given watershed's inputs. Let's start by understanding how global atmospheric circulation drives large-scale patterns in precipitation (8:45 min).

The Coriolis effect is important to determining the direction of transport of water in the atmosphere. It also creates the collision of air masses that controls the frontal systems that are the producers of substantial precipitation in climates like those of Montana (7:13 min).

Working from global to local scales, let's review the major drivers of the atmospheric uplift necessary to drive precipitation (9:17 min).

The physical state of water in precipitation is critical to predicting whether water might be stored in snowpack before being subjected to other watershed hydrologic processes. Let's review the conditions that control whether precipitation is likely to be in form of snow, rain, or mixed precipitation (6:33 min).

The SNOTEL network provides an excellent example of how precipitation is measured, both as snow and rain. We can use the basic equipment at a SNOTEL site to review the considerations of accurate measurement of precipitation in either form (8:49 min).

Once precipitation is measured at a gauge or a network of gauges, further work is necessary to properly aggregate and interpolate those point measurements to estimate precipitation over a whole watershed (18:50 min).

Graphs of precipitation over time are called hyetographs. Let's review some of the basics of hyetograph interpretation (7:11 min).

Probabilistic interpretations of hyetographs are heavily utilized in risk analyses in tandem with the probabilistic interpretations of hydrographs we have reviewed. We will discuss the application of storm duration frequency analysis in later modules about rainfall-runoff modeling, but lets review the basic concepts first (9:35 min).

Storage in snowpack has become a natural service that the human infrastructure of the western US and many communities in mountainous environments depends on. Let's take a closer look at snow data and the influence of the orographic effect using examples from some local SNOTEL gauges (3:14 min).

Snowmelt is a threshold driven process that is likely sensitive to relatively small changes in the patterns of late spring and early summer temperatures. Thus changes in climate are likely to have profound influence on changes in storage in snowpack. The key to understanding the drivers of snowmelt is to understand the details of the thermal energy budget of the snowpack (13:41 min).

Snow accumulation and melt are categorized by specific phases tied to the thermal state of the snowpack and the mechanical ability of the snowpack to retain liquid water. Calculating the energy needed to change that thermal state of the snowpack and generate snowmelt is a good opportunity to practice thinking through the concepts of energy budgets, specific heat, and latent heat (22:00 min).

The data necessary to categorize a full snowpack energy budget are seldom available, especially at whole-watershed scales. Though they need to be recalibrated for every watershed, more empirical temperature index models of snowmelt are reasonably effective when more detailed data are not available (3:02 min).

Study guide

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Study guides are designed to summarize the vocabulary, concepts, and mathematics learned in this module.

Readings from Dingman (3rd ed)

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A list of associated readings from Physical Hydrology by S. Lawrence Dingman (3rd edition)

Slides used in videos

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Laboratory preparation materials for this module

Link to online laboratory preparation materials

Astrum video on global circulation

A more detailed (and much higher production quality) video about global atmospheric circulation from the Astrum YouTube channel.

More visualizations of global circulation

A nice video from the Met Office (the UK's national weather service) about global circulation cells.

The influence of Pacific Ocean surface temperatures on precipitation

A NOAA web page with a description of ENSO (the El Niño - La Niña cycle)

Current NOAA diagnostic discussion on the likelihood of El Niño or La Niña conditions

Detailed SNOTEL Information

USDA-NRCS web page with description of the SNOTEL network

Video on the basics of SNOTEL sites

OnTheSnow video describing SNOTEL gauges