Hydrology BC ENV 3025

a moisture laiden air parcel rises, cools at dry adiabatic lapse rate (~1^oC/100m) (= no heat exchange with environment) until it reaches the dewpoint, at which point condensation occurs. After that, any further rise causes cooling at the moist adiabatic lapse rate (0.5 - 0.9^oC/100m), because of the released latent heat. (Fig).

storm track and total rainfall accumulation during a storm on June 27, 1995 based on radar measurements (Fig2.5)
current radar image
radar image of previous fieldtrip (Fig)

the record of hourly precipitation over time is called a hyetograph and shows that precipitation is organized into discrete storms (Figure 2.3, a station in North Carolina)
our ability to forecast this temporal variation even a few hours in advance is
if you examined all of the rainfall data for a given region, you would find an upper limit to

hydrologists apply a technique called frequency analysis to describe the temporal characteristics of precipitation
we assume that precipitation data are samples of a random variable characterized by a probablility density function
only mean annual precipitation appears to be normally (or Gaussian) distributed (Fig2.7)
if normally distributed, precipitation can be described by a mean and and a standard deviation (Fig2.6)
this information is useful to determine the exceedence probability (the probability that a certain annual precipitation value is exceeded in a given year) or the return period (the inverse of the exceedence probability).
If data are normal distributed, EXCEL's NORMINV function can be used to determine the value of a particular parameter (e.g. annual precipitation) for a particular exceedence probability and recurrence interval
determination of exceedence probablity using standard deviation, mean and the normal distribution. Use LGA/NY as an example (Fig2.6).

Evapotranspiration

Interception

process by which precipitation falls on vegetative surfaces (the canopy), where it is subject to evaporation (Fig2.8)
stemflow (flow along the stem), throughfall (the precip that has avoided interception or dripped off the canopy)
effects of different kinds of vegetation on capacity and runoff (Fig)
afforestation could increase evaporation losses by up to 50 to 100%, clearcutting increases runoff

evapotranspiration summarizes all processes that return liquid water back into water vapor
water needed and solar energy
of the water taken up by plants, ~95% is returned to the atmosphere through their stomata (Fig)
potential evaporation (PE), i.e. the evaporation rate given an unrestricted water supply - different from actual evaporation
how can the actual evapotranspiration be measured?

p = 10⁷+/- 5 *10⁵m³/y

r_si = 10⁹ +/- 1.5*10⁸ m³/y

r_so = 9.95*10⁸ +/- 1.5*10⁸ m³/y

ET = p + r_si - r_so

A control volume for energy conservation (Fig2.9), R: solar energy in, H: sensible heat flux out, El: latent heat flux out, G: sensible heat flux into ground
the energy balance for the control volume: dQ/dt = Rn-G-H-E_l
et = E_l / ( ρ_w λ_v)
if G, H, and changes in Q over time can be neglected:
et = Rn/ρ_w λ_v= 200 Wm^-2 / (1000 kg m^-3 * 2.5 10⁶ J kg^-1) = 0.7 cm day ^-1
in many case, we cannot neglet all the other energy fluxes, see for example measured energy fluxes for a field in CA (Fig 2.10)
it is not easy to measure the fluxes in the real world, Eddy Covariance measurements measures vapor pressure and vertical windspeed
there are lots of empirical equation describing evapotranspiration, example the Penman equation or combination equation (Fig)
keep in mind that the actual et depends on

The Atlantic Ocean is loosing water by evaporation (Fig), this loss is balanced by transport of ocean water from the Pacific ocean (1 Sverdrup = 10⁶ m³ s^-1, equivalent to the discharge rate of all rivers on Earth)

Jones, J.A.A. (1997) Global Hydrology. Addison Wesley Longman Ltd., Essex, UK, 399p