Modeling catchment scale hydrology

Soil physical characteristics, hydrological flow paths,
and the water balance

Predicting the hydrology of a catchment is challenging. Direct measurements of soil physical parameters are time-consuming and assumptions are made due to the spatial variation. Other parameters such as the evapotranspiration and (sub-)surface runoff are also difficult to measure.

This thesis focused on a small agricultural dominated catchment in southern Norway, containing 3 main soil types ranging from heavy clay to sand and 2 land uses, namely agriculture and forest. The existing combinations of land use and soil types each have different effects on the hydrology. Currently, it is unknown what the distribution between the surface and subsurface runoff are, and how much the evapotranspiration is. In trying to understand these parameters and water distributions the hydrological model DRAINMOD has been used in this thesis. Simulations have been performed with different values for the surface storage, drainage intensity, drain spacing, saturated lateral hydraulic conductivity, various soil temperature thresholds and evapotranspiration parameters.

Introduction

Climate change scenarios predict an increase in both temperature and precipitation and more frequent rainfall episodes with an increased intensity for Norway. Winters will become less severe and the number of freezing and thawing cycles might increase, resulting in less stable soil aggregates (groups of soil particles) and an increase in erosion . However, an increase in temperature can also mean a longer growing season. Analyses have been done about the potential effects of changing climate conditions in Norway. The predicted increase in temperature in Southeast Norway is 3.4 C for the period 2071-2100, compared to the years 1961- 1990, with 4.5 C in the winter season and 2.5C in the summer season. The predicted increase in precipitation
is 12.2 %, compared to the same period of 1961-1990, although not evenly spread throughout the year.

Agriculture

Norwegian agriculture is highly influenced by the winter season.

  • If temperatures stay below zero and snow accumulation occurs longer than usual, the sowing of crops is delayed, reducing the
    overall growing period and thus the yield.
  • Drainage during spring can drain the excess soil water and increase the growing period length. More intense drainage have been found to lead to earlier sowing times. Due to the drainage system, less water is present in the soil which means less ice formation during the winter period, which increases the infiltration capacity of the soil.
  • Also, when ice is present in the soil it needs to be melted by increasing the soil temperature, which takes time and thus decreases the length of the growing season. This fast drawdown of groundwater is needed in spring and autumn for farmers to be able to manage their fields, but also during periods of excess rainfall.

To deal with the effects of the changing climate a good understanding of the behavior of the drainage system is needed.

The upscaling from a field to catchment scale adds additional issues. A field rarely has a homogeneous distributed soil, a whole catchment even less so. There are often more land uses in a catchment scale than in a field scale, for instance, forestry or urban area. Field data collection is one method of gaining information on soil physical parameters, evapotranspiration and surface runoff, but is often
costly and time consuming. Models can help with this, by gaining additional insights in the local hydrology, but should not be a substitute for field work. Thus the issue arises of how to incorporate suitable soil physical parameters, from the field, in a model which represents a whole catchment in order to understand the hydrological processes.

Research goal

This research filled the knowledge gap of DRAINMOD’s suitability by evaluating temporal and spatial variability in water runoff, from different soil type and vegetation combinations and watershed scale, by modeling the hydrological processes and as such to get detailed knowledge about the water balance components.

Method

The method mainly consists of running many simulations with different values for:

  • Drain spacing
  • Soil physical parameters
  • Surface storage
  • Threshold temperatures

Among many others. Contact me if you would like more detailed information about this.

Conclusions

Of the simple water balance (P = ET + DR + RO), only data on the precipitation (P) and total runoff together (DR+RO) is present, at a catchment scale. The main challenge was to trace these streams back to their origin in the field (where different combinations of soil type, drains, slope etc. were present).

The main goal of this thesis was to assess if DRAINMOD is suitable for Norwegian catchments. Based on the results of simulations on  Skuterud, DRAINMOD is not the best option. This is because the model does not simulate enough surface runoff, considering that the catchment is hilly. The adjustment of the surface storage intended to compensate for this, but with no success.

This thesis was made possible thanks to the funding of IRIDA, JOVA and NIBIO